Visibility was reportedly poor, with dense fog concealing the top of the crane, according to eyewitness Quinn Murray, speaking to the BBC Breakfast News television programme.
The helicopter crashed to the ground in Wandsworth Road in Vauxhall and exploded into flames. Ambulances, fire engines and police were swiftly on the scene.
Two people died in the incident. One was the pilot, who had no passengers at the time; the other was a passer-by on the ground.
The Terex luffing job tower crane that was hit was working nearby for Brookfield Multiplex on the 52-storey St George Wharf Tower, set to be the country's tallest residential tower block and already at its full 185m height. The photo below shows the building before the crash.
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