Officers from the Metropolitan Police were alerted to the crash in Streatham Hill A23 at around 1.20pm today (December 14).
The cyclist, a male, was found injured at the scene and was taken to hospital in a critical condition.
The driver of the bus stopped at the scene and there have been no arrests.
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Enquiries into the circumstances surrounding the crash continue.
Road closures remain in place in the road headed northbound.
Multiple bus routes have been affected and will run on a diverted service, including the 57, 59, 109, 133, 137, 159, 201, 250, 333, and 417.
