Rush hour commuter services have been disrupted after reports a person was struck by a train near Clapham Junction.
Several platforms at the station were closed off on Friday afternoon.
The incident is causing disruption to South Western Railway services, which is expected to last until the end of the day. The train company said a separate trespass incident had also caused delays.
Some Southern and Gatwick Express services are also affected.
In advice to passengers, National Rail said: “The emergency services are dealing with an incident in the Clapham Junction area, meaning some lines are currently blocked.
“Whilst they safely attend to the incident, trains may be cancelled, delayed by up to 45 minutes or revised. Disruption is expected until the end of the day.”
South Western Rail said: “We received reports that a person was struck by a train between Vauxhall and Wimbledon.
“To allow our response teams and the emergency services to assist with the incident power to the track had to be switched off in the affected area. This brought all trains nearby to a stop.
“There was also a separate trespass incident at Clapham Junction that caused delays once the lines began to reopen following the initial incident.
“All lines have now reopened, but due to the nature of the incident trains are likely to be disrupted until the end of service.”
Southern said in a message to customers: “We are only able to stop at one of our two available platforms at Clapham Junction, in the direction of London Victoria.”
It said this is affecting services from Littlehampton, Reigate, Portsmouth, East Grinstead and Eastbourne to London Victoria.
A London Ambulance Service spokesperson confirmed it was called shortly after 3pm, saying it had also dispatched an air ambulance.
“We sent a number of resources to the scene, including an incident response officer, members of our Hazardous Area Response Team (HART), a medic in a fast-response car and two ambulance crews,” said a spokesperson.