A 20,000-pound construction platform collapsed and then crashed down 13 stories onto a busy downtown street Monday, killing three people and crushing cars stalled in midday traffic.

The collapse occurred about 1:20 p.m. on Boylston Street, which runs along the south side of Boston Common.

Witnesses said there was a terrifying rumble then crash of the platform lift system, which was set up atop a second building next to a 14-story building owned by Emerson College. The platform and scaffolding, which had been used to install a stone facade, were being dismantled around the 13th story when the collapse happened, said acting Fire Commissioner Kevin MacCurtain.

Two people who ended up in the street and one in a car were killed, Boston Fire Lt. David M. Pfeil said, while two other people were injured. The federal Occupational Safety and Health Administration was called to inspect the remaining scaffolding and assist in the investigation, said Boston Police Superintendent Robert Dunford.

John D. Macomber, president and CEO of Macomber Builders, the lead contractor hired by Emerson, said at a news conference the dead were believed to be two construction workers employed by subcontractor Bostonian Masonry and a passer-by.

He said the company was still investigating the accident.

"It looks as though one of the moving, movable scaffoldings fell off the side of the building out toward Boylston Street and down. We don't know why that happened yet. They're tied in laterally very well," Macomber said.

He said the procedure was a "typically very safe way to work."

"I had just walked through the spot where it fell when I heard this roar," said Dan Rofsky, 19, an Emerson freshman from New Jersey. "To turn around, after hearing this crash 30 feet away," he added, pausing to collect himself, "I just saw this cloud of dust and smoke."

The building — a dormitory and campus center — has been under construction for nearly two years, and was scheduled to open for the fall semester, said David Rosen, the school's vice president of public affairs. No students were injured.

"It's very distressing," Rosen said.

Brian Trimmer, assistant manager of Commonwealth Books, said he looked out the store's front window just as an eight- to 10-foot chunk of machinery crashed onto a mid-sized silver Honda that had been headed east on Boylston Street.

"I saw this large orange thing fall out of the sky on top of a car," Trimmer said. "It flattened it."

James Carney, an educational consultant, saw the falling piece of machinery barely miss a white car and crush the Honda in front of it. A cloud of dust rose as a construction worker screamed in pain.

On the sidewalk below his office, Carney saw another construction worker, who looked like he had fallen from the scaffolding above, lying motionless. A pedestrian who had been hit by debris wailed in pain.

When rescuers moved the piece of machinery off the Honda, Carney saw the lifeless body of a young man dressed in green hospital scrubs.

"It was ghoulish and awful," said Carney, chairman of Carney, Sandoe & Associates. "It was just a terrible, terrible day."

John Hynes was driving his BMW the opposite direction of the Honda, west on Boylston Street, when the platform and scaffolding started to tumble.

"You could see it coming down and then I started rolling forward. It started to hit my car, and then I sped up," said Hynes, a Boston resident and grandson of the former Boston Mayor John Hynes.

The debis damaged the roof of his car and smashed his rear windshield. Hynes, however, was not injured.

Macomber, based in South Boston, has been in business for more than 100 years and helped build Boston's Faneuil Hall Marketplace, according to the company's Web site.

The company's Web site also promotes an employee safety program, and employees a safety director.

The collapse occurred on a busy street that borders the Common just around the corner from the city's theater district. The area was crammed with students and passersby.

Mayor Thomas M. Menino looked somber after touring the accident site.

"It's terrible," the mayor said, shaking his head. "People just driving down the street in their cars, construction workers just doing their jobs — they never knew what hit them."