Violence at EDL founder Tommy Robinson's Sunderland book signing
Police were called in after trouble flared at a book signing in Sunderland by the English Defence League founder Tommy Robinson.
Violence broke out at Saturday's event, which was staged in an off-licence shortly before a Sunderland-Celtic match being held the same day.
Police attended Fletcher's News and Booze, where an 18-year-old man suffered a head injury and needed hospital treatment.
Twenty-one arrests were made in total.
Officers said the majority were unrelated to Mr Robinson's book signing and included people being drunk and disorderly, committing a breach of the peace and using illegal pyrotechnics.
Emergency services had been called to the off-licence in Hendon at about 13:40 BST.
The force said Mr Robinson was wearing a Glasgow Rangers shirt - the arch rivals of Celtic.
Footage posted on social media showed a flare and other objects being thrown towards the shop before fighting broke out on the street.
The pre-season match was organised to mark 20 years since the opening of Sunderland's Stadium of Light and 50 years since Celtic won the European Cup.
Assistant Chief Constable Helen McMillan said: "Unfortunately, we did have some incidents of disorder in and around Sunderland between Sunderland and Celtic fans and, while police dealt with them quickly, it is a shame these type of incidents happen at all."
Sunderland were beaten by Celtic 5-0.