Eurostar suspended all trains to and from St Pancras due to a trespasser draped in the St George's flag who climbed up on to a roof above the railway tracks.
People are still facing delays on the Eurostar after services resumed at around 9.45am.
"We strongly recommend you not travel this morning and please cancel your journey or exchange ticket free of charge," a Eurostar spokesman said earlier on Saturday.
It came after a night of disruption at St Pancras after the trespasser climbed onto the roof of the station at 7pm.
A 44-year-old man has now been arrested on suspicion of trespass and obstruction of the railway "having spent the night on the roof of St Pancras," British Transport Police said.
An image posted on social media on Saturday morning shows a person draped in the St George's flag standing on a roof above the tracks with police vans parked nearby.
Southeastern said that the overhead wires are not able to be recharged while the "person is above the high speed lines".
The statement said: "High speed trains will not run to and from St Pancras due to the ongoing trespass incident just outside the station.
"The person is above the high speed lines and as such the overhead wires can not be recharged."
Hundreds of people were affected by the disruption on Saturday, with one person saying they had "never seen St Pancras so busy".
"It took me five times as long to go from one side to the other," Kryssi wrote on Twitter.
"There's hundreds of people outside Eurostar and no one making room for people wanting to get past."
Police were initially called at around 7pm on Friday and said officers were "working to resolve the situation as soon as possible".
"Just after 7pm officers were called to a man in a precarious position at St Pancras," BTP said in an earlier statement.
"They, along with colleagues from other emergency services, are working to resolve the situation as soon as possible."
