THE helicopter crash was witnessed by many commuters who were on their way to work in the rush hour.
Mark Louis Sidney, who was heading for Vauxhall station, told Sky News: "I heard a loud bang, I looked up and saw bits of propeller and parts of the aircraft falling off it and then plummeting down. Then a plume of smoke went up.
"I called the emergency services. It happened in a matter of a seconds before it was on the ground. I thought 'Wait a minute, has this thing been shot out of the sky or what?'.
"I could see the top of the crane was shaking on the top of the building. It was very foggy so the helicopter probably couldn't see it."
Mr Sidney said the emergency response was "very professional" and there was "no delay".
Conservative MP Nicky Morgan was walking towards Vauxhall when the accident happened and said she thought it was a bomb explosion.
She told BBC News she realised something "dramatic" had happened because of the thick black smoke.
"Within a minute there were sirens, so you felt that the emergency services were immediately reacting, which was comforting. It is a terrible, unexpected tragedy.
"I wondered if it was a bomb explosion."
Eyewitness Steve Carslake told BBC Radio 5 Live that he saw a car explode with someone apparently trapped inside.
Mr Carslake said he had got out of a van in Mill Street and saw the aircraft hit the crane.
He said: "We heard a mighty bang, looked up and saw the helicopter just catch the edge of the crane.
"It didn't hit the tower block itself, it hit the crane and then just came down in a fireball.
"We ran round the corner.
"There was a great big lot of black smoke everywhere and there was a couple of cars.
"We heard someone was actually trapped in the car. We went to run towards the car and there was just a large explosion again.
"It was quite devastating to watch, I'm afraid.
"It was very, very sad to watch."
One witness told the BBC he heard the helicopter crash and feared it was a bomb.
He said: "I thought it was a bomb, I literally thought it was a bomb.
"It wasn't too foggy this morning about 7.30am, but then, in half an hour or an hour it was really foggy.
"It was crazy really."
One of the first firefighters on the scene said it was "absolute chaos", but revealed that the helicopter blaze was extinguished within 20 minutes.
Station Officer Bruce Grain said the helicopter had clipped the crane, spun out of control and crash-landed, hitting several vehicles.
"It burst into flames, which spread to adjacent buildings," he said.
"I could see smoke from a mile away. There was absolute chaos. The helicopter was still alight, part of the rotor blade was on the roof of nearby buildings.
"The fire was brought under control within 20 minutes. We were aware that the helicopter could have had a full tank of fuel, but we just used water."
Eyewitness and mother-of-five Sharon Moore, who lives on the Wyvil Estate, said she saw the aircraft slice through the crane "as if it was a piece of paper".
Ms Moore, 36, who was with her nine-year-old daughter Tiah, described hearing four bangs and watching two cars explode.
She said: "My daughter was so traumatised, it was so shocking. It sounded like a massive explosion, like something whining in the sky and then it just went bang
"You couldn't see a lot at first because of the fog. The helicopter did not seem to know which way to turn and then it just dropped, it sliced, screeching into the metal.
"It sliced it as if it was a piece of paper. It just ripped apart."
She went on: "I called the police and they got there in seconds. We could then see them trying to pull people out of the cars."
She described seeing what appeared to be body bags.
"We saw someone pulled out and they were covered."
She said the incident "like something out of an action movie".
"We saw the helicopter falling out of the sky, loads of smoke as it landed.
"It went bang. Another car went bang. There was lots of smoke and debris."
She later dropped her daughter at school where she was being comforted by staff.