Mayor Michael Bloomberg said at least four people, believed to be construction workers, have died and at least 10 people were injured in one of the city's worst construction accidents in recent memory.
"It is a tragic event," Bloomberg said.
The collapse created a virtual war zone on an affluent block on Manhattan's East Side: Cars were overturned and crushed. A huge dust cloud rose over the neighborhood. Rubble was piled several stories high.
An intensive rescue operation was under way to find anyone possibly trapped. One man was pulled from a collapsed townhouse 3½ hours after the building was crushed.
Fire Commissioner Nicholas Scoppetta said the rescue was "a painstaking hand operation, as we try to remove the rubble so we don't cause further collpase or injure anyone who may still be in that building." He said the operation would continue all night if necessary, including the use of search dogs and thermal-imaging and listening devices.
John PlaGreco, who owns Fu Bar in the crushed building, said he feared one of his employees was dead inside.
"Our bar is done," he said. "The crane crashed the whole building. If I wasn't watching a Yankees game, I would've come to work early and gotten killed."
About 19 of the planned 44-story condominium had been erected, and the crane was scheduled to be extended Saturday so workers could start work on a fresh story, said an owner of the company that manages the construction site.
A piece of steel fell and sheared off one of the ties holding it to the building, causing it to detach and topple, said Stephen Kaplan, an owner of the Reliance Construction Group.
"It was an absolute freak accident," Kaplan said. "All the piece of steel had to do was fall slightly left or right, and nothing would have happened."
Kaplan said the company had subcontracted the work to different companies and was not in charge of the crane. He wasn't sure whether any workers at the site were among those killed.
Neighborhood residents said they had complained to the city several times about the construction at the site, saying crews worked illegal hours and the building was going up too fast.
City Building Department records showed that on March 4, a caller told officials that the upper portions of the crane appeared to lack the proper number of safety ties attaching it to the building.
A city inspector visited the site and determined on March 6 that no violation was warranted.
Another call questioning the crane's safety was dismissed as unwarranted by another inspector in February.
On Saturday, nearby residents reported hearing a terrible roar as the structure detached from the condominium.
"First I heard a rumble, and it increased, and then it increased," said Bill Reilly, who lives a block away. "It continued building in strength until there was a final vroom! It shook the six-story brick building that I live in."
Witnesses reported a strong smell of gas in the area on 51st Street near 2nd Avenue. Gas utility Consolidated Edison said it shut off service to area buildings.
Ben Galati, 54, said he was in the basement of a high-rise apartment building across the street where he is a doorman.
"I heard a rumble outside. I said 'Let's get out of here!' And then the crane came down. A split second later, I heard an explosion," he said.