London Victoria is “at a standstill” because of a “major signal failure” during rush-hour.
Part of the station, the country’s second busiest, is currently closed due to overcrowding fears. Services are facing delays and cancellations until the end of Wednesday.
Southern Rail, which operates many of the services, is advising passengers not to travel from Victoria.
About 75m passengers passed through London Victoria last year.
Images posted on social media showed hundreds of passengers held on the station concourse, unable to catch Southern, Southeastern and Gatwick Express trains.
Thameslink services out of London Bridge have also been affected by the problems.
Peter Kyle, the Labour MP for Hove and Portslade, was caught up in the disruption. He described the central London hub as being “at a standstill”.
He tweeted: “I’m sorry to every passenger, I know there’s a lot more that needs sorting on this service, I’m fighting for that. You have been let down badly this evening.”
Network Rail apologised and advised people to find other routes if possible.