Commuters faced rush hour chaos as cancelled north-bound trains from St Pancras sent the station into "meltdown".
Images emerged on social media of furious passengers packed into carriages and stranded on overcrowded platforms as damaged overhead power lines at Luton left trains unable to run.
Govia Thameslink services faced delays and cancellations between St Pancras and Bedford, Luton, Sutton, Sevenoaks and Gatwick Airport.
East Midlands services were unable to run from the central London station towards Nottingham and Sheffield.
National Rail has warned passengers despite the overhead lines being fixed, a signalling problem has caused further delays.
Frustrated customers took to Twitter to discuss the situation.
Chris Widgery described “total chaos” at St Pancras.
He said: “It's not just things break down. It's that it's such total chaos afterwards.”
Annabel Grant added: “Another night of train disruption - stuck on a Thameslink trying to get north to Kings Cross.”
Katie Scott wrote: “Been waiting on Blackfriars platform for 50mins with constantly changing times for a train to Sutton ... what's happening?"
Others shared images of packed platforms at Blackfriars and Farringdon.
A statement posted on National Rail’s website said: “Damage to the overhead wires at Luton is causing major disruption to trains between Bedford and London St Pancras International.
“Trains are currently unable to run between Bedford and St Albans and as a result trains to and from London St Pancras International may be amended, cancelled or delayed by up to 90 minutes.
“Although Network Rail engineers are attending the problem, we expect disruption to continue until the end of service.”
Thameslink customers are advised to use London buses, Overground and Underground services.
East Midlands passengers are advised to use replacement bus services or London Midland trains between London Euston and Northampton, CrossCountry between Derby, Leicester, Birmingham and Peterborough and Virgin Trains East Coast and Virgin Trains on reasonable routes.
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