It comes after a video emerged, appearing to show the right-wing activist Tommy Robinson, real name Stephen Yaxley-Lennon, walking back and forth next to a man lying motionless on the ground.
Police said the man was arrested at Luton Airport shortly after 6.30pm, “following a notification that the man had boarded an incoming flight from Faro”.
“The man had been wanted for questioning after leaving the country to Tenerife in the early hours of 29 August following the incident at St Pancras,” officers said.
“He was arrested on suspicion of GBH (grievous bodily harm) and will now be taken to custody for questioning.”
Officers said they were called to the scene just after 8.40pm on Monday last week following reports of an assault.
A man was taken to hospital with serious injuries.
