A spectacular four-alarm fire blazed through the upper floors of the U.S. Postal Service headquarters in L'Enfant Plaza late last night and early this morning, sending clouds of thick black smoke billowing over Southwest Washington.

Firefighters said shortly before 1 a.m. that they were on the verge of subduing the blaze, which appeared to be one of the District's largest and most potentially damaging and difficult to control in months.

By a few minutes after midnight almost every available fire truck in the city had been sent to the scene at 475 L'Enfant Plaza SW, which is situated on 10th Street, a few hundred yards south of Independence Avenue and the Mall.

Officials said that about 190 firefighters using 48 pieces of equipment fought the blaze, which was first reported on the ninth floor of the building shortly after 11 p.m.

Based on initial accounts, all of the approximately three dozen people working in the gleaming 10-story glass-and-marble office building escaped without injury, but 16 firefighters were taken to a hospital for treatment of smoke inhalation and exhaustion. A number of other firefighters received first aid on the scene.

The cause of the fire was not immediately known, and no estimate of damage was available, but it appeared that smoke and fire damage to the upper three stories of the building was extensive. Damage to lower stories was expected from the heavy flow of water used to battle the blaze.

The building's 10th floor, where fire damage was believed severe, houses the offices of Postmaster General William F. Bolger, head of the Postal Service. The effect of the blaze on postal operations could not be readily assessed early this morning.

However, building manager Edgar Moody said he was pessimistic about prospects for opening the building for work today.

While scores of spectators -- many of them guests at the nearby L'Enfant Plaza Hotel -- gathered to watch, tongues of flame leaped from windows on the west side of the building's upper stories, and dense clouds of black smoke billowed into the night air.

With shrieking sirens of arriving fire trucks echoing off the walls of nearby buildings, firefighters already at the scene began clambering up aerial ladders in the glare of floodlights to direct streams of water into the blazing building.

Other firefighters wearing breathing masks went into the building to fight the fire at close quarters, as a police helicopter clattered overhead.

Traffic was halted on streets for blocks around the area, and the Southwest Freeway, which lies within yards of the building, was shut down.

Sgt. Thomas Davis of the Postal Service security force said he sounded an alarm after discovering the fire on his rounds in the building.

After descending a staircase from the 10th floor, he said, he opened the door leading to the ninth floor and "smoke just bulged out."

Fire officials said they received the first alarm about 11:25 p.m. In addition to security guards, the building was occupied at the time by about 20 members of the custodial force. About eight employes of the Public Broadcasting Service, which has offices in the building's basement, were also at work at the time.

Most workers left quickly after hearing the sound of the interior fire alarm. Fred Martin, 48, of Springfield, a PBS employe, said he was uncertain at first of the significance of the alarm. Then, he said, he looked out the window, and saw flames reflected in the windows of a nearby building.

Although few firm details were available this morning about the cause of the fire or the contents of the areas affected, one firefighter said it was believed that the blaze may have started in electrical equipment on the eighth floor. Another report indicated that the fire may have been concentrated in a series of ninth and 10th floor rooms in which records and supplies were stored.

About an hour was required to bring the blaze under control, according to accounts of fire officials.

Assessing the difficulties in fighting the upper-story blaze, Deputy Fire Chief Philip Matthews said "the biggest problem was location, magnitude and smoke. Very heavy smoke."