Thousands of farmers and rural workers have clashed with police in central China.
Demonstrators burned squad cars and attacked officers in the province of Hunan.
Local officials say the rioters were protesting about government corruption while human rights activists blame rising public transport costs for the violence.
Officials from the provincial government say they have arrested many of the protestors and are tracking down the organisers.
Police and rioters were injured in the violence and some were sent to hospital, though none was seriously hurt.
Poverty and inequalities between rural and urban areas have fuelled a series of demonstrations and riots across China.
The government says there were 23,000 "mass incidents" - protests, petitions and demos - last year.
Politicians have made repeated promises to bring prosperity to the countryside but China's 750 million peasant farmers are still treated as second class citizens.