Overcrowding at London Bridge railway station resulted in "life threatening chaos" at rush hour, the Rail, Maritime and Transport union (RMT) has said.
Passengers jumped over and crawled under ticket barriers on Tuesday night in an effort to avoid the crush on the concourse and platform in south London.
Network Rail said services were delayed by someone being hit by a train near Streatham Common.
It has apologised for the disruption to thousands of passengers.
Network Rail switched off the power supply to the trains between about 16:20 and 17:05 GMT to prevent further harm to a person who jumped on to the tracks near Streatham Common.
It said turning off the power had a knock-on effect on other services and added: "We're sorry this incident has caused the disruption it has."
It said a queuing system was put in place for safety reasons but demand was so high it closed the station doors on the advice of the transport police and only reopened them once the crowds inside had begun to disperse.
Network Rail and the train operator Southern said they would review the situation to see if any lessons could be learned.
RMT general secretary Mick Cash blamed cuts and a general lack of capacity on the trains for the "horrendous meltdown".
'Chaos must end'
He said: "The fragmentation and privatisation of our railways leaves train companies and Network Rail blaming each other while passengers and staff are left caught in the kind of life threatening chaos we have seen in pictures from one of London's major rail terminals last night.
"RMT is demanding action before this chaos becomes a tragedy."
London Mayor Boris Johnson said: "This chaos must end now. The scenes at London Bridge last night were completely unacceptable."
He said all sides had to stop bickering and called on Network Rail and the train operators "to get a grip" of the situation at London Bridge.
London Assembly Labour group transport spokeswoman Val Shawcross said it was a return to the "disastrous days of chaos" experienced earlier in the year because of the upgrade work to the station.