Long queues were visible at St. Pancras station after a man was arrested for trespass and obstruction of the railway. The European rail service advised passengers to change their travel plans following the incident.
The Eurostar rail service, which runs trains between London, Paris and Brussels, suspended all trains in and out of Britain for several hours after a 44-year-old man blocked the railway line on Saturday.
British Transport Police said the suspect was arrested for "trespass and obstruction" of the railway.
Local media showed images of a man carrying an English St. George's flag on the roof of the St. Pancras train station. The police said he had spent the night above the station, and that he was waving the flag in an apparent act of protest.
On Friday evening, police had said a man was "in a precarious position" at the London terminal.
The delay happened a day after thousands of protesters converged on the British parliament, many waving St. George and United Kingdom flags, in protest against Brexit being delayed.
In a statement posted later on Twitter, Eurostar said it services had resumed.
Eurostar has experienced several delays and other disruptions for weeks due to a strike by French customs officers, who are concerned about extra workload should Britain leaves the European Union.
shs/rc (AP, Reuters)