Tommy Robinson has been arrested by British Transport Police over allegations of assault at St Pancras International Station on 28 July.
The 42-year-old was detained at Luton Airport on earlier this evening after arriving from Faro, Portugal.
Police say the arrest follows an investigation into the assault, which left a man injured in the station concourse.
Robinson reportedly left the UK hours after the incident.
He was arrested on suspicion of grievous bodily harm and remains in custody for questioning.
Detectives are appealing for witnesses or anyone with information to assist their ongoing investigation.
