Elliott Tittensor was attacked as he came out of court after being fined for driving without insurance during a car accident which left a teenager in hospital.
The 20-year-old, who plays Carl Gallagher in Shameless, had just admitted the offence at Highbury Magistrates' Court when he was set upon outside by a grey-haired man in his 40s.
An onlooker said: "He was walking down the steps when a guy tapped him on the shoulder and just smacked him in the stomach all of a sudden. He came from nowhere, I don't know who he was."
Three policemen held the assailant on the ground and led him away down a nearby street.
The Metropolitan Police confirmed that a man has been arrested for affray in connection with Tuesday's incident.
Deborah Goodman, a publicist for Shameless, confirmed that Tittensor and his girlfriend, Skins actress Kaya Scodelario, were not injured in the attack. She said the pair were "really shocked" by what had happened.
Tittensor had pleaded guilty to driving his BMW without insurance during an accident which left 18-year-old Oliver Flint with permanent eye injuries.
The TV star was in the car with his girlfriend Scodelario, 18, in the early hours of August 15 when the incident happened outside a fast food restaurant in Kentish Town Road, north London.
District Judge Robin McPhee ordered Tittensor to pay a £750 fine and he was given six penalty points on his licence.