The incident happened at a three-story building at 34 Conselyea Street in the Williamsburg section, near Union and Metropolitan Avenue around 1:30 p.m.
Authorities say the three workers were pulled from the rubble with injuries. One, a man in his 40s, was rushed to Bellevue Hospital. He remains in critical but stable condition. Two others were taken to Woodhull Hospital in stable condition.
The fourth person injured refused medical attention.
Forensic engineers are on site trying to determine how the collapse happened, while the construction crew, covered in dust, looked dazed.
Some limped down Conselyea Street, surrounded by police, paramedics and firefighters.
"The fire department was there within three minutes," Deputy Assistant Fire Chief Frank Leonard said. "We found two people trapped in the building. One up to his waist, one completely covered."
"We heard a very loud noise and the whole thing started coming down," witness Maly Blomberg said. "There was a cloud of dust and stuff. There were two men standing at the door. One was holding a phone and shaking and the other was screaming that somebody was trapped."
Blomberg and her boyfriend snapped into action after the structure came down. She called 911 while he ran in the building.
The Department of Buildings confirms that China Perfect Construction was gutting the place, changing it from a three-story building to a four-story building with a penthouse.
"This gut renovation project included structural changes to the interior of the building," Buildings commissioner Robert Limandri said. "Our investigation is early, but we have preliminarily concluded that there were structural members that must have been compromised for the integrity of this building to collapse in this manner."
China Perfect Construction did not return a request for comment, but the buildings department is talking to company officials.