After a rally organized by the British Trades Union Congress, the National Union of Students and the University and College Union, a group of protesters broke away to march through the city. Twenty people were arrested by the police, who said that they were trying to incite violence. the rally, which drew around 3,500 people was against spending cuts and the coalition government. Police said that they main rally was a peaceful event and praised the organizers.
See "Twenty arrests after post-demo clashes in Manchester", BBC News Online, January 30, 2011