Officers responded to calls just after midday of a collision involving a route 131 bus and a pedestrian outside Tooting Broadway Station.
The London Ambulance Service and the London Air Ambulance also attended and the pedestrian involved has been taken to hospital with potentially life-threatening or life-changing injuries.
Tooting High Street has been closed in both directions whilst Garratt Lane is reduced to one lane westbound and eleven bus routes have been placed on diversion.
The impact to traffic is serious with congestion on all approaches and the footpaths are also closed at scene with pedestrians and cyclists having to seek alternative routes.
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