Migrant workers rioted for three days in a town in eastern China, officials and a rights group said, in a fresh sign of rumbling social unrest.
The protests began on 10 July in Kanmen in the coastal province of Zhejiang.
Workers - reportedly angered by a beating meted out to a colleague - attacked a police station for three successive nights.
The incident comes just weeks after a 30,000-strong crowd torched dozens of official buildings in Guizhou province.
They were protesting about an alleged cover-up over the death of a teenage girl. Police said the girl committed suicide but her family said she was murdered by the son of a local official.
One hundred people have been detained over the Guizhou riots, Chinese state media reported on Monday, including "39 members of local gangs".
The incident in Kanmen erupted after a migrant worker was beaten by a security guard, a Hong Kong-based human rights group said.
A crowd surrounded the police station after officers detained the worker, who had gone to file a complaint, the Information Centre for Human Rights and Democracy said.
Three nights of protests and rioting followed, a local official website confirmed.
"This was a grave crime of obstructing public security organs in carrying out their duties and assembling a crowd to attack state offices," a local police official was quoted as saying.
Three hundred military police arrived in the town on Sunday, another local official told the Associated Press news agency.
In recent years, more and more reports of local protests have emerged from China.
Corruption, land seizures, abuse of power by local officials and the widening gap between rich and poor are the common causes.
The incidents are of serious concern to Chinese authorities, who are keen to present an image of social stability ahead of the Beijing Olympic Games.
Officials have been told to ensure local protests do not mar the Games.