Two people were killed when a helicopter hit a crane at a building site in central London and plunged to the ground, police said.
Police did not say if the deaths were passengers and crew in the helicopter or people on the ground at the scene of the crash, an under-construction apartment block in Vauxhall on the banks of the River Thames.
The stricken aircraft cartwheeled to the ground and exploded into flames before crashing into a street during the rush hour.
Burning wreckage and aviation fuel covered the road as eyewitnesses reported seeing cars on fire and hearing people screaming.
London Fire Brigade said the crash happened in South Lambeth.
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Witnesses posted photographs of burning wreckage on social networking sites.
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."
Sarah-Beth Casey lives in an apartment near the incident.
She told Sky News: "When I heard the explosion - it was like a little earthquake.
"It was like a gas explosion. I looked up to see debris falling off the tower.
"I can't see any people on the ground, it seems to be a lucky escape. The police and ambulance response was incredible."
Erin Rogers was waiting at a bus stop outside Vauxhall station.
He told BBC News: "It was a bit surreal actually. I just had a coffee in my hand, I looked up, heard a bang and saw bits of crane debris falling to the floor.
"Then the helicopter was in flames. The rest of the people at the bus station were looking on going 'What was that?'.
"It's something I will never forget for a long time."
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.
Two people have been confirmed dead in the helicopter crash, Scotland Yard said on Wednesday.
Chris Yates, an independent aviation expert, told Sky News: "Helicopters are not supposed to come within 500 feet (152 metres) of any structure such as a high-rise building, so we don't know what caused the pilot to get quite so close.
"We don't know the circumstances he was operating in at the time - whether there was a problem with the helicopter itself, whether he misread his instructions or received false instructions from air traffic control."
