More than 20 football fans were arrested after trouble broke out ahead of a friendly match between Sunderland and Celtic.
Pictures on social media show a number of clashes in Sunderland, including one at a book signing in the city by English Defence League founder Tommy Robinson.
Northumbria Police said 21 arrests were made on Saturday afternoon, the majority of which were for drunk and disorderly behaviour, breach of the peace and the use of illegal pyrotechnics.
In one incident, an 18-year-old man needed hospital treatment for a head injury. Police said he has since been discharged and inquiries are continuing.
Celtic won 5-0 in the game, which was organised by sponsors to mark 20 years since Sunderland's Stadium of Light opened and 50 years since Celtic won the European Cup.
Assistant Chief Constable Helen McMillan 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."