A three-alarm fire in one of Capitol Hill's underground tunnels yesterday sent billows of acrid black smoke through vents at the city's main post office and other locations including the Supreme Court.

The blaze disrupted mail operations and tied up more than two dozen fire-fighting vehicles manned by more than 120 firemen for several hours.

Postal officials said their investigation indicated last night that no mail had been lost, and that only empty mail sacks were involved.

it also coincided with a White House ceremony at which District fire officials were helping to kick off fire prevention week.

Officials estimated the fire damage at between $20,000 and $30,000. It knocked out phone lines to the post office, but postal officials said they expected service to be restored by this morning.

The fire started shortly before noon in a 300-foot tunnel between the post office and the Government Printing Office on North Capitol Street when the roller in a mail conveyor belt jammed, officials believe.

The blaze engulfed mail sacks and electrical wiring and grew to hot that it "melted" concrete in the tunnel's ceiling, fire officials said. The tunnel is connected to a maze of utility tunnels that carry steam heat to the Capitol and other buildings on the Hill.

The conveyor belt involved is used primarily to carry copies of the Congressional Record and the Federal Register from the GPO pressroom to the post office for mailing.

Eddie Palmer, who works on the top floor of the post office building at North Capitol Street and Massachusetts Avenue, said, "I smelled smoke, but I just sat where I was (for about 30 minutes after the fire was detected) until somebody came through yelling fire. This is the first fire I've seen in 16 years on the job."

Employees of other buildings were not evacuated, since their structures - including the Supreme Court - got only wisps of smoke.

At the White House, District Fire Chief Burton W. Johnson and other officials gathered at about 12:45 p.m. while the fire still was being fought to make Vice President Mondale and honorary fire chief, according to a fire department spokesman.