A nearly 20-story-tall crane toppled backwards from a 25-story building in Manhattan, flattening several stories of a nearby building and killing at least four people, according to city officials.
At least 10 others were injured, and an unknown number may be trapped in the rubble, city officials said.
Initial reports were that between four and eight people were believed to be trapped, but city officials said it was premature for an accurate count for one building that was almost completely destroyed by the crane's impact.
Initial reports were that of the criticially injured at least two more people were likely to die.
New York City Mayor Michael Bloomberg said it appeared that all of the dead were construction workers, and he said the crane collapse was one of the worst construction accidents in the city's history.
"It is a tragic event," Bloomberg said. "Our hearts go out to all the victims and their families."
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.
One of the owners of Reliance Construction Group, the company that contracted for the crane for the 44-story condominum tower it was building, told The Associated Press that a piece of steel fell and cut one of the ties that held the crane to the building.
"It was an absolute freak accident," Reliance owner Stephen Kaplan said. "All the piece of steel had to do was fall slightly left or right, and nothing would have happened."
The collapse created a virtual war zone on an affluent block of Manhattan: Cars were overturned and crushed. A huge dust cloud rose over the neighborhood. Rubble was scattered everywhere and piled several stories high.
The construction site where the collapse occured had been plagued by numerous serious violations, Manhattan Borough President Scott Stringer told ABC Radio's Aaron Katersky.
Hundreds of rescue workers, dozens of fire trucks, and dozens more police cars and trucks, ambulances and emergency work crews rushed to the scene at Second Avenue and East 51st Street on Manhattan's East Side.
The heart of Manhattan was sealed off from traffic in the aftermath of the collapse.
Several of the injured appeared to be workers, though some were also reported to be passersby.
A witness told ABC News New York affiliate WABC that there was a large boom like an explosion when the construction site crane collapsed.
Ben Galati, a 54-year-old doorman at a high-rise apartment tower across the street, said he was in the basement when it happened, and ran for his life when he heard the structure smash into his building.
"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.
ABC News affiliate WABC in New York contributed to this report.