More than 20 bomb threats were made against federal buildings and offices in the District yesterday, causing the Supreme Court, the Department of Housing and Urban Development and two agencies to evacuate thousands of workers while police combed their buildings for explosives, officials said.

Threats were also received by a Northwest savings and loan and by the East Wing of the National Gallery of Art, where security guards cleared about 700 visitors from the museum's Treasure Houses of Britain exhibit for an hour.

No explosives were found in any of the 24 offices or buildings, which included the U.S. Capitol, two Senate office buildings and a House office building, the departments of State, Justice, Labor, Interior, Commerce and Energy, the FBI's Hoover Building and the IRS building, among others.

Officials said the threats were all made by telephone, most between 11:15 a.m. and 1:45 p.m., with the caller sometimes contacting the agencies directly and at other times informing various media that a bomb had been planted in a building. On at least six occasions, the caller identified himself as a member of "The People's Liberation Army." He did not explain the purpose of the threatened bombing, officials said.

"It appears that the person or persons responsible may have been a crank caller," the D.C. police department said in a statement. Police said they believe one person or group of people was responsible for all the threats.

The D.C. police and FBI are investigating the threats with other law enforcement agencies, such as the Treasury Department's Bureau of Alcohol, Tobacco and Firearms, the U.S. Capitol Police and the Federal Protective Service.

Law enforcement officials would not comment yesterday on whether the People's Liberation Army is an active group. A U.S. Capitol Police spokesman said that a man claiming to be a member of the group telephoned their headquarters about 11:30 a.m. and said a bomb would explode in the Capitol at noon.

The spokesman said that 15 minutes later a local radio station and a television station called and said they had been contacted by a man purporting to be a member of the PLA who said a bomb would explode in the Dirksen and Russell Senate office buildings.

In 1972, two PLA members were linked to the bombing of the Manchester, N.H., police and fire department headquarters and the planned bombing of President Nixon's New Hampshire primary campaign headquarters, according to news accounts published at the time.

John Rees, an expert on terrorist groups who is publisher of Information Digest, said the PLA was a left-wing group of about a dozen persons that was formed during the Vietnam War and was "in sympathy with the Vietcong and anxious to carry out 'anti-imperialist' actions in the United States."

Rees said the group was also tied to oil company boycotts in the late 1970s in Houston and on the West Coast. He said that there were no bombings related to the boycotts.

Officials said that the rash of threats stretched their resources to the limit as the D.C. police bomb squad's nine members and four dogs raced from one office building to another. A police official said it was "an unusually high number of bomb threats."

"The threat is a disruptive tactic more often than a warning," the official said. "Somebody is getting the results they want. They are creating havoc . . . . A lot of people suffered for one jerk to get his jollies."

A spokeswoman for the Supreme Court said a man telephoned the chambers of one of the justices on an unpublished line about 3:45 p.m. and said a bomb would explode in the building at 6 p.m. She said employes of the court were notified of the threat and advised to leave the building by 5:30. She said the evacuation was not mandatory.

At the General Accounting Office, 441 G St. NW, more than 3,000 workers hurriedly evacuated the building when an alarm sounded about 1 p.m.

"We got our coats and marched outside," said Laura McBride, a mail clerk who sat outside the building rubbing her gloved hands together trying to keep warm. "I'm not scared but it sure is annoying."

Federal Protective Service officers blocked off traffic in both directions on G Street between Fourth and Fifth streets while the GAO building was searched. Many employes huddled in small groups along a wall across the street at the Metro building and talked until they were allowed back inside about 2:30 p.m..

About 4,000 employes of the Department of Housing and Urban Development were evacuated from their building at 451 Seventh St. SW for about an hour after a caller telephoned HUD Secretary Samuel R. Pierce Jr.'s office about 11:20 a.m. and warned, "We have a nice bomb wrapped up for you."

In addition, about 1,700 workers were evacuated for an hour from Judiciary Plaza, 450 Fifth St. NW, which houses the offices of the Securities and Exchange Commission and private firms, an SEC spokeswoman said.

Dale Bruce, spokesman for the General Accounting Office, which manages most federal buildings, said the threats came in randomly, with the caller sometimes giving a specific time that a bomb would explode.

Bruce said that when a threat comes in, an official from the agency that has received the threat consults with other officials and the Federal Protective Service before deciding whether to evacuate. "If there's any question in the official's mind about employe safety, the building is evacuated. We have an err-on-the-side-of-safety policy."