Police have been battling to control thousands of demonstrators taking part in a protest organised by a controversial right-wing group.
The English Defence League (EDL), an organisation which opposes "radical Muslims" and Sharia law, organised the rally in Victoria Square, Bolton, Lancashire.
A counter-demonstration by Unite Against Fascism (UAF) was also being held, and hundreds of police officers were trying to keep the two factions separate, aided by police dogs.
Some 31 UAF protesters and three from EDL have been arrested, police said.
Assistant Chief Constable Garry Shewan, from Greater Manchester Police, who is leading the policing operation, said: "There have been unwarranted attacks on police lines that have resulted in injuries.
"This is not a peaceful protest and we are facing a lot of hostility. We will take swift action when confronted with disorder."
There are around 2,000 UAF protesters in the square but the number of EDL demonstrators is much lower, police said.
Riot police and mounted officers armed with batons were trying to keep the crowds in check in front of the town hall.
Two members of the public were taken to a shop for treatment to minor injuries after being caught up in the trouble, police said.
Assistant Chief Constable Shewan said: "The number of arrests already made is a clear indication that this is not a peaceful protest and some demonstrators are determined to cause trouble. The actions of some demonstrators is resulting in injuries to others. This is not acceptable."
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