A helicopter crashed in central London and plunged to the ground in a ball of flames on Wednesday, with police saying it had hit a crane on top of a building.
Fire engines and police were at the scene of the crash near the River Thames at Vauxhall, where a huge plume of smoke billowed into the sky.
Paul Ferguson, who was working in an office near the incident, told BBC News: "There was a flash and the helicopter plunged to the ground. It exploded and you can imagine the smoke coming out of it."
Witnesses posted photographs of burning wreckage on social networking sites.
Unconfirmed early reports said one person was on board, the pilot.
London's Metropolitan Police said the helicopter was not one of its aircraft. "We had a call at approximately 8am this morning and we are dealing with an incident," a police spokesman said.
The crash was causing long tailbacks on roads during the morning rush hour.