A cyclist is in a critical condition after being struck by a bus.
The man, believed to be in his 20's, was hit by the vehicle in St John's Hill, Battersea, just before 4pm this afternoon.
A spokeswoman for the London Ambulance Service: "We sent a single responder in a car, one ambulance crew, London Air Ambulance and a duty officer.
"He was taken to a priority to the major trauma centre at St George's Hospital.
"He went in a ground ambulance, escorted by the air ambulance doctor."
Witnesses expressed their shock on social networking websites.
Lee Donaghy wrote: "What's going on in Clapham Junction? Police everywhere, even helicopters circling."
Rob Brown said: "Loads of sirens and air ambulance trying to land near Clapham Junction."