UNITED STATES
                       SECURITIES AND EXCHANGE COMMISSION
                             Washington, D.C. 20549

                                  SCHEDULE 13D
                                 (Rule 13d-101)

                               (Amendment No. 20)

    INFORMATION TO BE INCLUDED IN STATEMENTS FILED PURSUANT TO RULE 13d-1(a)
                    AND AMENDMENTS THERETO FILED PURSUANT TO
                                  RULE 13d-2(a)

                            Prime Group Realty Trust
- --------------------------------------------------------------------------------
                                (Name of Issuer)

                      Common Shares of Beneficial Interest,
                            par value $0.01 per share
                            -------------------------
                         (Title of Class of Securities)

                                    74158J103
                                    ---------
                                 (CUSIP Number)

                                  Joseph Macnow
                              Vornado Realty Trust
                               888 Seventh Avenue
                            New York, New York 10019
                                 (212) 894-7000
- --------------------------------------------------------------------------------
            (Name, Address and Telephone Number of Person Authorized
                     to Receive Notices and Communications)

                                 With a copy to:
                                William G. Farrar
                                 Alan Sinsheimer
                             Sullivan & Cromwell LLP
                                125 Broad Street
                            New York, New York 10004
                                 (212) 558-4000

                                February 10, 2004
             -------------------------------------------------------
             (Date of Event Which Requires Filing of This Statement)

If a filing person has previously filed a statement on Schedule 13G to report
the acquisition that is the subject of this Schedule 13D, and is filing this
schedule because of Rule 13d-1(e), 13d-1(f) or 13d-1(g), check the following box
[ ].

The information required on the remainder of this cover page shall not be deemed
to be "filed" for the purpose of Section 18 of the Securities Exchange Act of
1934, as amended (the "Exchange Act"), or otherwise subject to the liabilities
of that section of the Exchange Act but shall be subject to all other provisions
of the Exchange Act.

                         (Continued on following pages)
                               (Page 1 of 9 Pages)

- --------------------                                       ---------------------
CUSIP NO. 74158J103                                            PAGE 2 OF 9 PAGES
- --------------------                                       ---------------------
- --------------------------------------------------------------------------------
 1.   NAME OF REPORTING PERSON: Vornado Realty Trust
      I.R.S. IDENTIFICATION NO. OF ABOVE PERSON (ENTITIES ONLY):
      22-1657560
- --------------------------------------------------------------------------------
 2.   CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP
                                                                     (a)  [X]
                                                                     (b)  [ ]
- --------------------------------------------------------------------------------
 3.   SEC USE ONLY

- --------------------------------------------------------------------------------
 4.   SOURCE OF FUNDS:
      AF

- --------------------------------------------------------------------------------
 5.   CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS IS
      REQUIRED PURSUANT TO ITEMS 2(d) OR 2(e)                              [ ]
- --------------------------------------------------------------------------------
 6.   CITIZENSHIP OR PLACE OF ORGANIZATION
      Maryland
- --------------------------------------------------------------------------------
               7.   SOLE VOTING POWER
  NUMBER OF         0
   SHARES      -----------------------------------------------------------------
BENEFICIALLY   8.   SHARED VOTING POWER
  OWNED BY          3,972,447
    EACH       -----------------------------------------------------------------
 REPORTING     9.   SOLE DISPOSITIVE POWER
   PERSON           0
    WITH       -----------------------------------------------------------------
               10.  SHARED DISPOSITIVE POWER
                    3,972,447
- --------------------------------------------------------------------------------
11.   AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING
      PERSON
      3,972,447
- --------------------------------------------------------------------------------
12.   CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES
      CERTAIN SHARES                                                      [ ]
- --------------------------------------------------------------------------------
13.   PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)
      16.8% as calculated in accordance with Rule 13d-3(d)(1)
- --------------------------------------------------------------------------------
14.   TYPE OF REPORTING PERSON
      OO (real estate investment trust)

                                    -2 of 9-

- --------------------                                       ---------------------
CUSIP NO. 74158J103                                            PAGE 3 OF 9 PAGES
- --------------------                                       ---------------------
- --------------------------------------------------------------------------------
 1.   NAME OF REPORTING PERSON: Vornado Realty L.P.
      I.R.S. IDENTIFICATION NO. OF ABOVE PERSON (ENTITIES ONLY):
      13-3925979
- --------------------------------------------------------------------------------
 2.   CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP
                                                                     (a)  [X]
                                                                     (b)  [ ]
- --------------------------------------------------------------------------------
 3.   SEC USE ONLY

- --------------------------------------------------------------------------------
 4.   SOURCE OF FUNDS
      WC

- --------------------------------------------------------------------------------
 5.   CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS IS
      REQUIRED PURSUANT TO ITEMS 2(d) OR 2(e)                              [ ]
- --------------------------------------------------------------------------------
 6.   CITIZENSHIP OR PLACE OF ORGANIZATION
      Delaware
- --------------------------------------------------------------------------------
               7.   SOLE VOTING POWER
  NUMBER OF         0
   SHARES      -----------------------------------------------------------------
BENEFICIALLY   8.   SHARED VOTING POWER
  OWNED BY          3,972,447
    EACH       -----------------------------------------------------------------
 REPORTING     9.   SOLE DISPOSITIVE POWER
   PERSON           0
    WITH       -----------------------------------------------------------------
               10.  SHARED DISPOSITIVE POWER
                    3,972,447
- --------------------------------------------------------------------------------
11.   AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING
      PERSON
      3,972,447
- --------------------------------------------------------------------------------
12.   CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES
      CERTAIN SHARES                                                      [ ]
- --------------------------------------------------------------------------------
13.   PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)
      16.8% as calculated in accordance with Rule 13d-3(d)(1)
- --------------------------------------------------------------------------------
14.   TYPE OF REPORTING PERSON
      PN

                                    -3 of 9-

- --------------------                                       ---------------------
CUSIP NO. 74158J103                                            PAGE 4 OF 9 PAGES
- --------------------                                       ---------------------
- --------------------------------------------------------------------------------
 1.   NAME OF REPORTING PERSON: Vornado PS, L.L.C.
      I.R.S. IDENTIFICATION NO. OF ABOVE PERSON (ENTITIES ONLY):
- --------------------------------------------------------------------------------
 2.   CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP
                                                                     (a)  [X]
                                                                     (b)  [ ]
- --------------------------------------------------------------------------------
 3.   SEC USE ONLY

- --------------------------------------------------------------------------------
 4.   SOURCE OF FUNDS:
      AF
- --------------------------------------------------------------------------------
 5.   CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS IS
      REQUIRED PURSUANT TO ITEMS 2(d) OR 2(e)                              [ ]
- --------------------------------------------------------------------------------
 6.   CITIZENSHIP OR PLACE OF ORGANIZATION
      Delaware
- --------------------------------------------------------------------------------
               7.   SOLE VOTING POWER
  NUMBER OF         0
   SHARES      -----------------------------------------------------------------
BENEFICIALLY   8.   SHARED VOTING POWER
  OWNED BY          3,972,447
    EACH       -----------------------------------------------------------------
 REPORTING     9.   SOLE DISPOSITIVE POWER
   PERSON           0
    WITH       -----------------------------------------------------------------
               10.  SHARED DISPOSITIVE POWER
                    3,972,447
- --------------------------------------------------------------------------------
11.   AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING
      PERSON
      3,972,447
- --------------------------------------------------------------------------------
12.   CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES
      CERTAIN SHARES                                                      [ ]
- --------------------------------------------------------------------------------
13.   PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)
      16.8% as calculated in accordance with Rule 13d-3(d)(1)
- --------------------------------------------------------------------------------
14.   TYPE OF REPORTING PERSON
      OO (limited liability company)

                                    -4 of 9-

                        AMENDMENT NO. 20 TO SCHEDULE 13D
             RELATING TO THE COMMON SHARES OF BENEFICIAL INTEREST OF
                            PRIME GROUP REALTY TRUST

         Vornado Realty Trust ("Vornado"), Vornado Realty L.P. (the "Operating
Partnership") and Vornado PS, L.L.C. ("Vornado PS" and, collectively with
Vornado and the Operating Partnership, the "Reporting Persons") hereby amend
their Statement on Schedule 13D filed with respect to the Common Shares of
Beneficial Interest, par value $0.01 per share ("Common Shares"), of Prime Group
Realty Trust, a Maryland real estate investment trust (the "Issuer"). This
Amendment No. 20 to Schedule 13D filed on February 11, 2004 ("Amendment No. 20")
should be read in conjunction with the Statement on Schedule 13D of the
Reporting Persons filed on November 2, 2001 (the "Initial Schedule 13D"), as
amended by Amendment No. 1 to Schedule 13D of the Reporting Persons filed on
November 20, 2001, by Amendment No. 2 to Schedule 13D of the Reporting Persons
filed on December 19, 2001, by Amendment No. 3 to Schedule 13D of the Reporting
Persons filed on December 21, 2001, by Amendment No. 4 to Schedule 13D of the
Reporting Persons filed on December 27, 2001, by Amendment No. 5 to Schedule 13D
of the Reporting Persons filed on January 15, 2002, by Amendment No. 6 to
Schedule 13D of the Reporting Persons filed on January 31, 2002, by Amendment
No. 7 to Schedule 13D of the Reporting Persons filed on February 7, 2002, by
Amendment No. 8 to Schedule 13D of the Reporting Persons filed on February 20,
2002, by Amendment No. 9 to Schedule 13D of the Reporting Persons filed on March
29, 2002, by Amendment No. 10 to Schedule 13D of the Reporting Persons filed on
April 17, 2002, by Amendment No. 11 to Schedule 13D of the Reporting Persons
filed on April 19, 2002, by Amendment No. 12 to Schedule 13D of the Reporting
Persons filed on May 1, 2002, by Amendment No. 13 to Schedule 13D of the
Reporting Persons filed on May 14, 2002, by Amendment No. 14 to Schedule 13D of
the Reporting Persons filed on July 3, 2002, by Amendment No. 15 to Schedule 13D
of the Reporting Persons filed on December 23, 2002, by Amendment No. 16 to
Schedule 13D of the Reporting Persons filed on May 27, 2003, by Amendment No. 17
to Schedule 13D of the Reporting Persons filed on June 13, 2003, by Amendment
No. 18 to Schedule 13D of the Reporting Persons filed on June 18, 2003 and by
Amendment No. 19 to Schedule 13D of the Reporting Persons filed on July 30, 2003
(as so amended, the "Schedule 13D"). This Amendment No. 20 amends the Schedule
13D only with respect to the items listed below. Unless otherwise indicated, all
capitalized terms used but not defined herein shall have the meaning ascribed to
them in the Initial Schedule 13D.

         Pursuant to Rule 13d-4 under the Exchange Act, each of the Reporting
Persons declares that the filing of this statement shall not be construed as an
admission that such Reporting Person is, for the purposes of Section 13(d) or
13(g) of the Exchange Act, the beneficial owner of any Common Shares.

         Any disclosures made herein with respect to persons other than the
Reporting Persons are made on information and belief after making appropriate
inquiry.

Item 2.  Identity and Background.

Item 2 is hereby amended solely to replace Schedule I, previously a part of this
Schedule 13D, with Schedule I attached to this Amendment No. 20 and to

                                    -5 of 9-

replace those responses to Item 2 of Schedule 13D that referenced Schedule I
with the statements below.

         (a), (f) The name and citizenship of each Trustee and executive officer
of Vornado are set forth in Schedule I hereto and are incorporated herein by
reference.

         (b) The principal business address of each Trustee and executive
officer of Vornado is set forth in Schedule I hereto and incorporated herein by
reference.

         (c) The present principal occupation or employment of each Trustee and
executive officer of Vornado and the name, principal business and address of any
corporation or other organization in which such employment is conducted are set
forth in Schedule I hereto and incorporated herein by reference.

         (d), (e) During the last five years, none of the Reporting Persons and,
to the knowledge of each of the Reporting Persons, none of the persons listed on
Schedule I hereto (i) has been convicted in a criminal proceeding (excluding
traffic violations or similar misdemeanors) or (ii) has been a party to a civil
proceeding of a judicial or administrative body of competent jurisdiction and as
a result of such proceeding was or is subject to a judgment, decree or final
order enjoining future violations of, or prohibiting or mandating activities
subject to, federal or state securities laws, or finding any violation with
respect to such laws.

Item 4.  Purpose of the Transaction.

Item 4 is hereby amended by adding the following paragraphs at the end thereof:

         On February 10, 2004, Vornado PS transmitted written notice to the
Issuer pursuant to Section 3 of the Registration Rights Agreement requesting
that the Issuer register for resale under the Securities Act of 1933 all
3,972,447 Common Shares held by Vornado PS. The proposed plan of distribution
provides for possible sales of Common Shares from time to time under a wide
variety of different methods.

Item 5. Interest in Securities of the Issuer.

         No transactions in the Common Shares were effected by the Reporting
Persons during the past 60 days, the date of execution of the Reporting Persons'
most recent filing, except to the extent that such transactions may be deemed to
have occurred as described in Item 4.

Item 7. Material to be Filed in English

         1. Letter, dated February 10, 2004, requesting registration of Common
Shares.

                                    -6 of 9-

                                   SIGNATURES

         After reasonable inquiry and to the best of my knowledge and belief,
each of the undersigned certifies that the information set forth in this
amendment to Schedule 13D is true, complete and correct.

Dated: February 13, 2004

                                            VORNADO REALTY TRUST

                                            By: /s/ Joseph Macnow
                                               ---------------------------------
                                               Name: Joseph Macnow
                                               Title: Executive Vice
                                                      President-Finance
                                                      and Administration,
                                                      Chief Financial
                                                      Officer

                                            VORNADO REALTY L.P.

                                            By: VORNADO REALTY TRUST,
                                                its general partner

                                            By: /s/ Joseph Macnow
                                               ---------------------------------
                                               Name: Joseph Macnow
                                               Title: Executive Vice
                                                      President-Finance
                                                      and Administration,
                                                      Chief Financial
                                                      Officer

                                            VORNADO PS, L.L.C.

                                            By: VORNADO REALTY L.P.,
                                                its sole member

                                            By: VORNADO REALTY TRUST,
                                                its general partner

                                            By: /s/ Joseph Macnow
                                               ---------------------------------
                                               Name: Joseph Macnow
                                               Title: Executive Vice
                                                      President-Finance
                                                      and Administration,
                                                      Chief Financial
                                                      Officer

                                    -7 of 9-

                                   SCHEDULE I

Name*                         Present Principal Occupation or Employment
- ----                          ------------------------------------------

Steven Roth                   Chairman of the Board and Chief Executive Officer
                              of Vornado.

Michael D. Fascitelli         President and a Trustee of Vornado.

Robert P. Kogod               Trustee of Archstone-Smith Trust.
(Trustee of Vornado)

David Mandelbaum              A member of the law firm of Mandelbaum &
(Trustee of Vornado)          Mandelbaum, P.C., 80 Main Street, West Orange, New
                              Jersey 07052.

Stanley Simon                 Owner of Stanley Simon and Associates, management
(Trustee of Vornado)          and financial consultants, 70 Pine Street, Room
                              3301, New York, New York 10270.

Robert H. Smith               Trustee and Chairman of Charles E. Smith
                              Commercial Realty Division of Vornado; Trustee of
                              Archstone-Smith Trust.

Ronald Targan                 President of Malt Products Corporation of New
(Trustee of Vornado)          Jersey, a producer of malt syrup; principal
                              business address: Malt Products Corporation of New
                              Jersey, 88 Market Street, Saddle Brook, New Jersey
                              07663.

Richard West                  Director or Trustee of Vornado, Vornado Operating
(Trustee of Vornado)          Company, Alexander's Inc., Bowne & Co., Inc. and
                              various investment companies managed by Merrill
                              Lynch Asset Management, Inc. or Hotchkis and
                              Wiley, both affiliates of Merrill Lynch & Co.

Russell B. Wight, Jr.         Trustee; A general partner of Interstate
(Trustee of Vornado)          Properties (real estate and related investments).

David R. Greenbaum            Chief Executive Officer of the New York Office
                              Division of Vornado.

Joseph Macnow                 Executive Vice President-Finance and
                              Administration and Chief Financial Officer of
                              Vornado.

Melvyn Blum                   Executive Vice President - Development Division of
                              Vornado.

                                    -8 of 9-

         *Unless otherwise indicated above, the business address of each person
listed above is: c/o Vornado Realty Trust, 888 Seventh Avenue, New York, New
York 10019. All of the Trustees and executive officers of Vornado are citizens
of the United States of America.

                                    -9 of 9-

                                                                       Exhibit 1
                                                                       ---------

                               VORNADO PS, L.L.C.
                               888 SEVENTH AVENUE
                            NEW YORK, NEW YORK 10019

February 10, 2004

Via Federal Express
- -------------------

Prime Group Realty Trust,
   77 West Wacker Drive, Suite 3900,
      Chicago, Illinois 60601.
         Attention: James F. Hoffman

               Re:  Registration Rights Agreement
                    -----------------------------

Ladies and Gentlemen:

         Reference is made to that Certain Registration Rights Agreement, dated
as of the 17th day of November, 1997 (the "Registration Rights Agreement"),
among Prime Group Realty Trust, Prime Group Realty L.P., Primestone Investment
Partners, L.P. and the other parties named therein. All capitalized terms used
herein, but not defined herein, shall have the meanings assigned to them in the
Registration Rights Agreement.

         We are an Investor and we hereby request, pursuant to Section 3.1 of
the Registration Rights Agreement, that Prime Group Realty Trust effect the
registration under the Securities Act of all of the 3,972,447 Common Shares
owned by us. We may sell such Common Shares from time to time, at market prices
prevailing at the time of sale, at prices related to market prices, at a fixed
price or prices subject to change or at negotiated prices, by a variety of
methods. Attached to this letter as Exhibit A is a more detailed plan of
distribution that should be included in the prospectus relating to such Common
Shares.

Prime Group Realty Trust                                                   -2-

         Please don't hesitate to call Joseph Macnow (201-587-1000) or our
attorneys Gary Israel (212-558-4005) and Bill Farrar (212-558-4940) if you have
any questions or if you need any assistance or information regarding the
registration process.

                                        Very truly yours,

                                        VORNADO PS, L.L.C.

                                        By:  Vornado Realty, L.P.,
                                                  its sole member

                                        By:  Vornado Realty Trust,
                                                  its general partner

                                        By:  /s/ Michael D. Fascitelli
                                           -------------------------------------
                                           Name:   Michael D. Fascitelli
                                           Title:  President

cc:  Wayne D. Boberg, Esq.
     (Winston & Strawn)

     Gary Israel, Esq.
     William Farrar, Esq.
     (Sullivan & Cromwell LLP)

                                                                       EXHIBIT A
                                                                       ---------

                                                            PLAN OF DISTRIBUTION

         The selling shareholder(s), and their pledgees, donees, transferees or
other successors in interest, if any, may offer and sell, from time to time,
some or all of the common shares covered by this prospectus. We have registered
the common shares covered by this prospectus for offer and sale by the selling
shareholder(s) so that those shares may be freely sold to the public by them.
Registration of the common shares covered by this prospectus does not mean,
however, that those shares necessarily will be offered or sold. We will not
receive any proceeds from any sale by the selling shareholders of the
securities. See "Use of Proceeds". We will pay all costs, expenses and fees in
connection with the registration of the common shares, including fees of our
counsel and accountants, and listing fees: We estimate those fees and expenses
to be approximately $________ million. The selling shareholder(s) will pay all
underwriting discounts and commissions and similar selling expenses, if any,
attributable to the sale of the common shares covered by this prospectus and all
registration fees payable to the SEC.

         The selling shareholder(s), including their pledgees, donees,
transferees or other successors in interest, if any, may sell the common shares
covered by this prospectus from time to time, at market prices prevailing at the
time of sale, at prices related to market prices, at a fixed price or prices
subject to change or at negotiated prices, by a variety of methods including the
following:

          o    in privately negotiated transactions;

          o    through put or call transactions related to the common shares;

          o    through short sales of the common shares;

          o    through broker-dealers, who may act as agents or principals;

          o    in a block trade in which a broker-dealer will attempt to sell a
               block of common shares as agent but may position and resell a
               portion of the block as principal to facilitate the transaction;

          o    through one or more underwriters on a firm commitment or best-
               efforts basis;

          o    directly to one or more purchasers;

          o    through agents; or

          o    in any combination of the above.

         In effecting sales, brokers or dealers engaged by the selling
shareholder(s) may arrange for other brokers or dealers to participate.
Broker-dealer transactions may include:

          o    purchases of the common shares by a broker-dealer as principal
               and resales of the common shares by the broker-dealer for its
               account pursuant to this prospectus;

          o    ordinary brokerage transactions; or

          o    transactions in which the broker-dealer solicits purchasers.

         At any time a particular offer of the common shares covered by this
prospectus is made, a revised prospectus or prospectus supplement, if required,
will be distributed which will set forth the aggregate amount of common shares
covered by this prospectus being offered and the terms of the offering,
including the name or names of any underwriters, dealers, brokers or agents, any
discounts, commissions, concessions and other items constituting compensation
from the selling shareholder(s) and any discounts, commissions or concessions
allowed or reallowed or paid to dealers. Such prospectus supplement, and, if
necessary, a post-effective amendment to the registration statement of which
this prospectus is a part, will be filed with the SEC to reflect the disclosure
of additional information with respect to the distribution of the common shares
covered by this prospectus.

         In connection with the sale of the common shares covered by this
prospectus through underwriters, underwriters may receive compensation in the
form of underwriting discounts or commissions and may also receive commissions
from purchasers of common shares for whom they may act as agent. Underwriters
may sell to or through dealers, and such dealers may receive compensation in the
form of discounts, concessions or commissions from the underwriters and/or
commissions from the purchasers for whom they may act as agent.

         Any underwriters, broker-dealers or agents participating in the
distribution of the common shares covered by this prospectus may be deemed to be
"underwriters" within the meaning of the Securities Act of 1933, and any
commissions received by any of those underwriters, broker-dealers or agents may
be deemed to be underwriting commission under the Securities Act of 1933.

         Pursuant to the terms of a registration rights agreement, we have
agreed to register the common shares covered by this prospectus for resale. We
have also agreed in the registration rights agreement to pay all the fees and
expenses relating to the offering, other than any registration fees, any agency
fees and commissions or underwriting commissions or discounts or any transfer
taxes incurred in connection with the resales, and have agreed to indemnify the
selling shareholder(s) and any underwriters against certain liabilities,
including those arising under the Securities Act of 1933.

         Some of the common shares covered by this prospectus may be sold in
private transactions or, if available, under Rule 144 under the Securities Act
of 1933 rather than pursuant to this prospectus.