A number of buildings have been evacuated after 200 square metres of scaffolding collapsed on to a north London street in heavy winds.
London Fire Brigade said the measure was taken as a precaution as two fire engines and three rescue units attended the scene in Pond Street, Hampstead Heath.
Staff at the nearby Royal Free Hospital were told not to go home because of the "major incident".
Video posted to social media showed the scaffolding strewn across the road as emergency services urged people to step back from the cordon.
There were no immediate reports of any injuries. London Ambulance Service said it was preparing an update on the incident.
A London Ambulance Service spokesperson said: "We were called at 2:45pm today (7 March) to reports of an incident on Pond Street, Hampstead.
"We sent an ambulance crew, an incident response officer, a single responder in a car and an advanced paramedic to the scene. We also dispatched London's Air Ambulance and members of our Hazardous Area Response Team.
"We are working closely with other members of the emergency services and remain at the scene."
A witness told the Camden New Journal: "I was going up towards to Hampstead and we stopped outside M&S – thank god.
Hilary Barnes said: "As we were waiting there this whole thing, it goes up the whole of the building, it started to very slowly go outward and then it buckled. It was extraordinary.
"There was an awful lot of screaming. Ambulances were charging down there within seconds. It looked like the wind called the polythene and the scaffolding came away.
"I am sure there will be casualties as it is so busy, with the schools and the Royal Free. There is no way any traffic is coming through there for a while."