Officers were called to the scene just after 8.40pm on Monday following reports of an assault.
A man was taken to hospital with serious injuries which are not thought to be life-threatening.
“Detectives are continuing to work closely to progress the investigation and bring the man into custody for questioning,” a statement reads.
It comes after footage emerged purporting to show right-wing activist Tommy Robinson walking back and forth next to a man lying motionless at St Pancras.
A concerned onlooker calls for help before Robinson shouts: “You saw him. He came at me bruv, you saw that, he came at me”.
