Daniel Toledo heard "a big noise" while on his way to the nearby Vauxhall station. Though Toledo didn't see the helicopter at first, he took out his camera in time to shoot exclusive aftermath video for CNN.

"I look up, after the helicopter down," said Toledo. "Big noise again. The helicopter is gone."

Nicky Morgan, a member of parliament, heard the impact from the nearby Vauxhall station.

"There was an enormous bang and clouds and clouds of thick, black smoke," she told CNN. "It was very surprising, not what you expect to see."

Despite the confusion, people remained calm, she said.

Christopher Jen was at Vauxhall Tube station on his way to his marketing job when he spotted smoke rising nearby.

"We could hear all the sirens and you could smell the smoke as the train went past the site," he said in a submission to CNN's iReport. "The station announcers did tell us that there was a helicopter crash at Vauxhall and the engineers had to go check the tracks before we could leave."

Reporters allowed through the cordons to the deadly crash scene say little can be seen other than a dark spot where the helicopter hit the ground.

A Berkeley Group spokesman said the firm was giving its full support to the emergency services after its crane was hit.

"Our thoughts at this time are with the friends and families of those killed in this tragic incident," he said.

Firefighters are now working with contractors to make the mangled crane safe.

Prime Minister David Cameron thanked the emergency services for their "rapid and professional response."

Poor visibility

Many helicopters follow the Thames as a flight route through the city. This helicopter appeared to be flying farther south than usual, Whipps said.

No cause has yet been given for the crash, but visibility was poor in London on Wednesday morning, with fog and a very low cloud ceiling.

Chris Yates, an aviation security expert, told CNN it was likely there would be questions about whether the helicopter should have been flying in these conditions.

Under safety regulations, tall buildings must have navigation lights on top to make them visible to low-flying aircraft, he said.

Helicopters are not supposed to fly within 500 feet of tall structures in central London, he said.

The London Heliport is near the crash site, on the banks of the Thames, at Battersea.

The Air Accidents Investigation Bureau, part of the UK Department for Transport, has sent a team to investigate the crash.