Police are appealing for witnesses to come forward after a man was pulled from the Thames on Saturday morning close to Waterloo Bridge.
Officers were called to Waterloo Bridge shortly before 8am following reports that a man had fallen into the river.
Boats from the Metropolitan Police's marine policing unit and the Maritime and Coastguard Agency attended and the man was pulled from the water within a few minutes. He was then taken to Victoria Embankment.
London Ambulance Service and London's Air Ambulance attended and treated the man, who is believed to be aged 70.
He was taken to an east London hospital where he is in a life-threatening condition.
Officers from Lambeth Police are investigating and have appealed for any witnesses to the incident to come forward.
In particular, they would like to speak to a man who was on Waterloo Bridge at the time of the incident.
Anyone with information should call 101 and quote CAD reference 2091.
At this time, officers are treating the incident as non-suspicious.