No trains were running to or from St Pancras International after a trespasser took to tracks and climbed a structure near the station on Friday night.
The disruption saw all trains stopped at the major terminus at this point, while power to over head lines had been shut off.
A statement from British Transport Police said: "Just after 7pm officers were called to a man in precarious position at St Pancras. They, along with colleagues from other emergency services, are working to resolve the situation as soon as possible. Thank you to everyone whose journeys have been disrupted for your patience."
While an update from National Rail said: "A trespasser on the railway in the London St Pancras International area is causing major disruption to and from this station. Trains may be cancelled, delayed by up to 60 minutes or revised."
Passengers have been advised to take alternative routes where possible.
Southeastern Railway, writing on Twitter, said: "They have climbed up on a structure and we along with the police take their safety seriously."
Another message added: "At present no trains are running from St Pancras."
The disruption is expected until the end of the day.
Emergency services are dealing with the incident.
National Police Air Service tweeted that it was responding to the incident with police.
Passengers have been advised to seek guidance from their travel providers on alternative routes and also compensation.
