British police say it was ''something of a miracle'' that many more people were not killed when a helicopter crashed into a crane on a construction site in central London while trying to avoid bad weather.
The crash, which killed two people including the helicopter's pilot, happened about 8am (9pm NZT), just metres from the busy Vauxhall train station. Up to 15 people were injured, one critically.
The pilot, named as Pete Barnes, 50, was well known in the industry and regarded as highly experienced.
He flew a helicopter in films including Die Another Day and Saving Private Ryan, was previously an air ambulance pilot and taught others to fly, including Formula One driver Lewis Hamilton.
The helicopter's blades hit the crane's cab and debris rained down on the street after the peak-hour collision.
Helicopter fuel spewed across the road and ignited, setting cars alight and producing plumes of smoke that could be seen for kilometres. One man was rescued from a burning car.
Witnesses described the chaotic scene as ''absolute madness''.
"People were screaming. It was madness, absolute madness. There was smoke to the eyeballs. The explosion was like a bomb. There were three cars on fire. One was completely disintegrated, you couldn't see it at all," witness Craig Marchland told the London Evening Standard.
Police said the repercussions of the crash could have been much worse.
''It was something of a miracle that this was not many, many times worse,'' police Commander Neil Basu said.
Barnes was the only person in the helicopter. It is believed that the crane driver was late for work.
Witnesses reported seeing the helicopter rocking and shaking from side to side before it crashed. Another said the burnt aircraft narrowly missed a train as it crashed to the ground.
''There was a big boom when the helicopter hit the crane, with bits and pieces dropping on the pavement, then another boom when it crashed on the ground down the road,'' said truck driver Ray Watts, who had been returning to a vehicle parked at the tower's security gate when the accident happened.
The helicopter had departed from Redhill Aerodrome, south of the city, for Elstree in the west, and sought a diversion to London Heliport in Battersea, two miles from the crash site, because of ''poor weather,'' Phil Wright, senior air traffic controller at Redhill, said in an interview.
London Heliport received a request from controllers at Heathrow airport to accept the flight, but never made contact with Barnes, it said.
It is believed Barnes was trying to land the helicopter on a helipad because of the foggy conditions.
An unnamed construction worker told The Guardian: ''I looked up and I saw the crane was disintegrating.
''The helicopter was flying towards us and went flying off the building. It was the first time I'd seen a helicopter fly this side of the crane - they usually fly along the river. I ran for my life. It is the most horrific thing I've ever seen in my life.''
British Prime Minister David Cameron said rules for helicopter flights over central London needed to be examined in light of the crash.
London mayor Boris Johnson echoed Cameron's comments.
''Our thoughts are very much with the deceased, their families and those who have been injured in this terrible accident. Clearly there will be questions in the course of the next few days and weeks about how it happened and what we can do to make sure it doesn't happen again,'' he said.
''What is inevitable is that we'll want to review all our policies, we'll want to look at the way we illuminate tall buildings, the way cranes are illuminated, to make sure nothing went wrong in this case and make sure nothing goes wrong in the future.
''It doesn't take a great deal to imagine what could have happened had that helicopter crashed into a bus or a heavily occupied building.''