The emergency services are responding to reports of a helicopter crash in south London.
The London Fire Brigade said it had received multiple calls of a helicopter hitting a crane in Vauxhall.
Ex-BBC producer Paul Ferguson said the helicopter "plummeted straight into the ground". People have reported seeing a pall of smoke in the area.
Police said they first received calls at 08:00 GMT and the London Ambulance Service said it was also at the scene.
Transport for London has confirmed there has been an incident.
Mr Ferguson added: "The building the helicopter hit is shrouded in mist."
He said the crane at the top of the building was now hanging down the side of the building.
'Lots of smoke'
Witnesses reported seeing debris on the ground.
Commuter Sherna Noah described seeing a "large plume of dark grey smoke" as she crossed Battersea Bridge at about 08:00 GMT.
She said: "I was coming across the bridge and basically I could see a few cyclists on the bridge looking towards the water.
"I looked over to see what they were looking at and could see a pall of grey smoke coming from the south side.
"You could see a large plume of dark grey smoke."
The BBC's Ross Hawkins said at least four ambulances and many police cars were at the scene.
He said Wandsworth Road near Vauxhall station is closed, with apparent debris on the ground, and a crane at the top of a new high rise development appears to be damaged.
Witnesses wrote about what they had seen on Twitter.
Belinda Moore wrote: "Just cycled in and saw lots of smoke over by Vauxhall, gather it's a helicopter crash, saw it flying low over Chelsea Bridge thought it odd."
Quin Murray said: "Helicopter just hit a crane and crashed a few metres in front of me in Vauxhall. Completely shaken."
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