Two men were badly injured when scaffolding around Hammersmith Town Hall collapsed on Wednesday evening.
Dozens of police, paramedics, firefighters and an air ambulance were called to the site in King Street, west London, at about 17:45 BST.
The men were taken to a major trauma centre "as a priority" but their condition has since stabilised.
The Metropolitan Police said the case had been passed to the Health and Safety Executive to investigate.
A police spokesman added the pair were still being treated in a central London hospital.
⚠️ROAD CLOSURE⚠️ King Street (W6) is temporarily closed to motorists between Dalling Rd and Hammersmith Broadway. Emergency services are attending to an incident in the area. Please avoid King St and the surrounding area where possible. We will update once King St reopens.
Residents nearby heard a huge bang. Radomir Grkovic, who lives opposite Hammersmith Town Hall, said the ground "shook like an earthquake".
He said: "I was in the flat. I just heard banging and it just shock. It was like an earthquake. It shook twice. It shook and then a couple of seconds later it shook again.
"I looked through the window and I couldn't see anything. All I know is that something fell."
Joyce Frimley, 81, who was asked to evacuate her flat, said: "There was a banging and then the place shook. It made me jump all day. It could have been a bomb."
Erick Dos Santos, who was working across the road at the time, said: "We had the door open. We heard a massive banging noise which sounded like something had fallen from quite a high distance."
Romano Dublin-Wilson, whose home backs on to the Town Hall site, said: "You can see part of the structure is bent. I think a crane fell on the structure. The right side is bent - it must have fallen on that."
The housing project is being completed by A2Dominion, on behalf of Hammersmith and Fulham Council, and a spokesperson said their "thoughts and best wishes are with all those involved, particularly the two men who have been injured", adding that the "immediate priority" was supporting the ongoing investigations.
The council closed King Street to motorists, but has since reopened the route.
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