Pimlico Tube station has been evacuated and the VIctoria line has been suspended due to a casualty on the tracks.
Emergency services rushed to the station on Monday afternoon.
Transport for London said there was no service running between Brixton and Victoria station.
Tickets are being accepted on London buses, TfL said.
Pictures taken at the scene show British Transport Police and London Fire Brigade at the scene.
An air ambulance also landed on Vauxhall Bridge, which was closed between Millbank and Albert Embankment.
TfL has since said the bridge has reopened, but traffic remains slow-moving northbound.
Richard Gowers wrote on Twitter: "Massive police/fire/ambulance presence at Pimlico station. And Vauxhall Bridge is closed for this air ambulance helicopter to park."
A spokesman for British Transport Police said: "We're at Pimlico station following a casualty on the tracks.
"This incident is not currently being treated as suspicious.
"We're working with TfL to resume Tube services as soon as possible."
