Video footage shot on a mobile phone showed an entire road blocked by burning wreckage and aviation fuel. The side of a building on one side of the street was also damaged by the flames.
Passers-by stood watching as the wreckage burned. A motorcycle was seen lying on its side in the road where it was abandoned.
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.
“It may be that on this misty morning the lights on nearby St George Tower weren’t on and it moved and clipped the edge of the crane and lost control.”
Witness Chris Matthison told BBC News: “There was some damage to the crane. The top of the nearest building is steeped in mist and difficult to see.”
He added: “I heard a very unusual dull thud, then there was silence.”
The aircraft is understood to be an AgustaWestland AW109, a lightweight, twin-engine helicopter with eight seats.