A smoky five-alarm blaze erupted tonight inside a mid-Manhattan office tower, and firefighters were searching the 42-story building for people possibly trapped inside, fire officials said.

The fire at the Westvaco Corp. building at 299 Park Ave., near 49th Street and across the street from the Waldorf-Astoria hotel, started at 7:38 p.m. and was brought under control at 10:23 p.m., officials said. Fire Department spokesman John Mulligan said the blaze was confined to the building's 20th floor. He described the fire as "tremendous."

A police department spokesman said about 100 people, mostly firefighters, were known to be injured and that extra ambulances from all five boroughs were still being called to the scene. Two firefighters were reported seriously injured on the 17th and 18th floors of the building, and rescue units were trying to bring oxygen tanks up to those floors.

Fifteen persons -- 12 firemen and three civilians -- were taken to Bellevue Hospital for treatment of smoke inhalation, according to hospital administrator Peter Schectman.

Schechtman said the civilians had been on the building's 25th floor and escaped down the stairs.

Robert Ash, a fire department spokesman, said firemen were searching the tower for people who may have been trapped, following reports that people were stuck on the 32nd floor.

The blaze shot flames out of windows and spewed smoke that could be seen blocks away. Police cordoned off the area between 47th and 51st streets and Madison and Lexington avenues because of falling debris and panes of glass, and heavy smoke.

"It's on the 20th floor," said fireman Bob Otley as he emerged from the building. "Seems to be kind of a whole wing that's going pretty good . . . It's a very cutting smoke; it closes your throat off."

He described the burning area as full of "plastic chairs, foam rubber, and we don't know what . . ."

"It's bad smoke," agreed firefighter Bill Andreaccio, who made it to the 21st floor before choking smoke forced him and several colleagues back down.

"It was so bad there we just had to bail out," he said. "We couldn't see. We couldn't breathe, and I just couldn't believe how hot it was."

Marc Rosenbaum, a lawyer working late in his 36th-floor office, said he and about 15 colleagues escaped by walking down the stairs.

"You could smell the smoke through the vents," he said. "It smelled like an electrical fire. When I passed the 20th floor, the fire exits weren't open. Either they were all overcome by smoke or there was just nobody in there."

"Everything was very orderly," said a man who had been in a 32d-floor office when he saw flames licking up from below his window.

Forty fire companies, including 150 firefighters, responded to the blaze, according to Mulligan. Firemen knocked out windows above the blaze on all sides of the building to try to remove the smoke.

The tower houses the offices of numerous laws firms and large corporations.

The blaze broke out less than three hours after a rush-hour fire at the Hyatt Hotel, under construction on 42nd Street next to Grand Central Station.