Violence broke out at an off-licence where ex-English Defence League founder Tommy Robinson was holding a book signing as trouble flared up before a Sunderland-Celtic football match.
Police attended Fletcher’s News and Booze in Sunderland as an 18-year-old man suffered a head injury and needed hospital treatment.
Northumbria Police said it was among series of incidents where 21 people were arrested across the city, with the majority of arrests for drunk and disorderly, breach of the peace and the use of illegal pyrotechnics.
Before the incident, Robinson was pictured at the off-licence wearing a Rangers shirt, Celtic’s arch rivals.
He had earlier posted a four-minute attack aimed at Celtic fans on Facebook where he talked about the club’s fans showing solidarity with Palestine, adding: “Palestine hates you.”
He said: “All these girls who have got their cleavage out and their Celtic tops … take your cleavage, take your Celtic top and walk through Gaza and show me your solidarity, because you won’t have any. You’ll have the biggest shock of your life. So stop hating on me.”
Footage of the incident was captured on social media.
The pre-season match was 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, Northumbria Police said:
“Fans come here to enjoy the football; many with young children and we here at Northumbria Police do all we can to ensure they enjoy the day whichever team they support.
“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 types of incidents happen at all.”