The collapse occurred at 867 E. 95th St., near Avenue D, just before 3:15 p.m., the FDNY said.
Of the five people who were working on the building, two suffered serious injuries but were expected to survive, FDNY Deputy Chief Stephen Moro said.
The collapse happened when the floor of the building — a three-story, two family home that was under construction — was overloaded with concrete blocks and other materials and then caved in, the Department of Buildings said.
The DOB issued a full stop work order on the site to continue investigating.
Fernella Lewis, 59, who lives across the street, said she saw a man standing on the building's plywood floor as it caved in, who then plummeted two stories to the ground.
“He had a huge gash in his head, and he couldn’t move his leg. I think his leg was broken. Blood was all over his face," said Lewis, who is a certified nurse's assistant.
"He was in a lot of pain, but he was very quiet, brave about it... I helped him until the EMS came and took over. It was really very frightening for everybody."
Froza Parvin, who is listed as the building's owner on the permit, raced to the scene of the collapse.
"I have no idea," Parvin said when asked what had happened. "I'm going there now."