Several dozen firefighters used heat-sensing equipment and microphones to search for survivors in the rubble of the three-story building in the working-class district of Tottenham. Others sifted by hand through the heap of brick, wood and metal left when the front of the building collapsed just after 8 a.m.
Police said five people were unaccounted for, and witnesses reported hearing voices from beneath the debris.
One witness, Elizabeth Sheehan, said she heard an "almighty rumble" and saw the building collapse in a hail of bricks and a cloud of dust.
"I could hear people screaming inside," she told Sky television news. "I shouted that we were going to get help for them."
A construction crew was working on the building two stories of apartments above a row of shops when the front of the structure caved in.
"Renovation work was being carried out at the time, but we can't confirm at the moment whether that was the cause," Tony Agar, spokesman for the London Fire Brigade, told Sky news.
Emergency crews cordoned off the area, which is close to the stadium of the Tottenham Hotspur soccer team.
Police said a man, a woman and a baby girl, believed to be residents of the building, were rescued and were taken to a hospital with minor injuries. Another woman was put on a stretcher but was also believed to have suffered minor injuries, the London Ambulance service said.
"We are trying to locate noises. We did receive a sound, but we cannot confirm it was a voice. It may just be the result of the collapse," Agar said.
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