Tear gas, set off by an accidental explosion in a weapons storage room, swept through the District of Columbia's police headquarters yesterday, causing an extraodinary disruption of police and other city government operations.

Almost 2,000 choking, teary-eyed city employees were evacuted from the city's Municipal Center, some on ladders and hydraulically operated "cherry pickers." Nonemergency police operations were suspended. The city government 's telephone switchboard was shut down. Tax officials ruhsed from their offices, leaving vaults containing checks and cash temporarily unlocked.

Almost 100 firefighters, police and other city employees were reported to have required medical treatment for tear-gas inhalation. Many were given aid by medical workers stationed outside the Municipal Center at 300 Indiana Ave. NW., where police headquarters and other city agencies are housed.More than 20 persons - mainly firefighters and police - were taken to local hospitals. None was reported to be seriously injured.

Hundreds of police rifles, shotguns and tear-gas launchers and thousands of rounds of ammunition were destroyed in the explosion and fire. One initial police estimate put damage to the equipment alonw at $300,000 or more. The storage room, near the lobby at the Indiana Avenur entrance to the Municipal Center, also was wrecked.

The tear gas that spread quickly through the six-story building's long corridors appeared likely to have been mixed with some harsher, more potent pepper gas. Police said pepper gas is normally kept in the weapons storage room, although they expressed uncertainty about whether any was stored there yesterday.

The city's acting police chief, Burtell M. Jefferson, insisted in an interview that no disruption in emergency police operations had occurred. "We still had-operational capabilities," said Jefferson, who will officially be installed as chief today. "We're got a back-up system."

Police dispatchers, who answer emergency telephone calls and dispatch squad cars when crimes occur, are stationed in the Municipal Center. When tear gas seeped into their fifth-floor room, Jefferson said, the dispatchers move temporarily to the Municipal Center's roof, where they answered calls with special walkie-talkies. They were soon given gas masks, he added, and returned to normal operations.

The Municipal Center is the adminstrative headquarters for the city's police department and for a number of special police branches, including homicide, robbery and other investigative squads. It also provides offices for city tax offices, the motor vehicles agency, the mayor's command center and the city government switchboard.

The shutdown of the city's switchboard because of the tear gas explosion only partly disrupted phone service for government workers. City employees could still be reached by phone through their direct-dial numbers. They could not be reached, however, through the city government's main phone number. In addition, city emplayees could not make long-distance phone calls without the switchboard.

The explosion occured at 1 p.m. in a heavily secured storeroom maintained by the police department's civil disturbance unit, which deals with riots and other emergencies. Chief Jefferson said that the police department would consider moving the storage room to another location to avoid another such incident. Tear gas canisters have previously gone off accidentally at police headquarters, but according to city officials, none has ever led to evacuation of the Municipal Center.

According to accounts given by police and fire officials, the initial blast occurred when a rocket-style tear gas grenade accidentally detorated while three policemen were rearranging ammunition in the storage room after a recent inventory. The canister exploded, they said, in a canvas bag containing about 10 other tear gas grenades. Officer Michael Gonzales, who was holding the canvass bag, was not seriously injured, though he suffered mild tear gas burns on his face.

The tear gas grenade's explosion triggered what officials termed a "chain reaction." Other cannisters started going off. The rapid chain of explosions generated heat and quickly resulted in a fire. Gonzales said in an interview that he and his colleagues sought to put out the fire with fire extinguishers, but found they could not. Eventaully, the fire and explosions led to detonations of 38 caliber and shotgun ammunition stored in the room. Apparently no one was hit by the exploding bullets, partly because the room is enclosed by steel doors and thick concrete walls.

Tear gas - both CN and the stronger CS - spread quickly throughout the building. Within minutes it had even penetrated a specially enclosed science laboratory on the Municipal Center's top floor - a lab that performs health and environmental studies, such as tests of water and milk quality, for the city's human resources department.

"The stuff suddenly came barreling down the hall," David I. Gale, the lab's administrator, said in an interview later. "We have our own glass dooor but the seals didn't hold." The lab also has its own ventilation system to prevent fumes from substances undergoing study from escaping into other parts of the building. "But," Gale added, "nothing can stop tear gas."

Gale and about 50 other laboratory workers climbed onto a sixth-floor balcony, from which they were eventually lowered - about five at a time - in the bucket of a fire department "cherry picker." Gale said he was uncertain hoe much damage the tear-gas caused to laboratory experiments and equipment.

Others made less dramatic exits. Many, especially those in parts of the building far removed from where the explosion took place simply walked down stairways after getting their first whiffs of tear gas. A few police and other city employees, whose offices are on the building's goroung floor, climbed out windows. Some, higher up, were led out on fire department ladders.

Gloria Kirby, a civil defense aide in the major's command center, was monitoring police and fire department radios when the blast took place. She heard about it on the radio shortly before she smelleed the gas, she recalled later in an interview. Wearing a gas mask, Kirby began helping others from the building. "This was my first time," she remarked.

The three-alarm weapons room fire posed unusual problems for the Fire Department. More than 100 firefighters were brought in, along with ambulances from the city and from Montgomery and Prince George's counties. Though the blaze itself was apparently confined to the storeroom, a room measuring about 35 by 60 feet, it kept firefighters at work for mor than two hours.

"The problem was getting to it," the city's acting fire chief, Jefferson W. Lewis, said later. The exploding tear gas and ammunition endangered firefighters. Some also were burned by tear gas - and possibly pepper gas - fumes. In additon, firefighters confronted an enormous evacuation task. Last night, military specialists were called in to help dispose of the damaged ammunition.

Many of the city government workers who were evacuated from the Municipal Center were told to go home early. it remained unclear last night whether their offices would be fit for normal operations today. The police department, however, shifted squads normally stationed at the Municipal Center to other locations. The Fire Department, a spokesman said, called in some 200 extra firefighters to offset manpower shortages elsewhere in the city stemming from the Municipal Center fire. They will be given compensatory time off, the spokesman said.

The stench of tear gas was no confined to the Municipal Center itself. It pervaded the surrounding neighborhood, where most local court buildings are situated. Spectators stood about, holding tissues, gloves and scarves over their noses and mouths.

The building's rapid evacuation caused some security gaps. According to senior city tax officials, finance and revenue workers hurried from their Municipal Center offices, leaving vaults unlocked, checks strewn on their desks and, in some instances even their purses. They did, however, lock their doors behind them, the official noted. Later, several tax officials, wearing gas masks, were sent back into the buildingto shut the vaults and secure the city's money.