An off-duty D.C. police officer was robbed of his service revolver and wristwatch during the course of a holdup in a Southwest Washington dentist's office where the officer was having a tooth repaired, D.C. police said.

After tying up the officer, the dentist and two employes, the holdup men fled with about $140 in cash. No drugs were stolen. Robbery investigators said four dentists' offices have been robbed in Washington within the last 13 days.

The robbery occurred at 10:45 a.m. in the office of Dr. Daniel Howard at 641 G. St. SW. Police said a man, armed with a handgun, entered the basement office and forced a receptionist to open the door leading into the examination area. A second man, who witnesses said appeared to be a juvenile, joined the gunman.

After robbing one of the employes of $15, the men surprised the doctor in the examination room and robbed the officer of his watch and .38-caliber service revolver. The doctor had no money on his person. The gunman than forced the doctor upstairs into the living quarters occupied by his wife and child. Cash totaling about $125 was taken from the wife's purse and the family was taken downstairs to join the other captives being held in the office.

Police said three other dentists' offices have been robbed since Jan. 21. One is in the 4300 block of Wheeler Road SE and the other two in the 4100 block of Connecticut Avenue.