Firefighters were called to West Heath Close in Hampstead after reports of a partial building collapse.
A 6m rolled steel joist weighing approximately 4 tonnes had collapsed onto construction workers. Nobody was imprisoned.
London Ambulance Service crews treated four people on the scene.
As a precaution, firefighters made the scene safe and established a hazard zone.
The Brigade was called in at 11.14am, and the incident was over by 12.09pm. Three fire engines from West Hampstead, Kentish Town, and Hendon were dispatched.
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