Last Wednesday, a migrant worker from Sichuan province named Zhang Zhongfu sought to apply for a temporary resident permit, but an argument broke out and he was beaten by police, the Hong Kong-based Information Centre for Human Rights and Democracy said.

Xinhua also reported the incident last night. Under the headline 'Hundreds of migrant workers attack police office in E. China', Xinhua said the riot lasted for three days and left three officers injured.

Xinhua did not say whether anyone was arrested, but it confirmed the human rights group's report as to why the riot had broken out.

Sichuan is a major source of migrant labour to the rest of the country, and Zhejiang is a centre of private enterprise, which has made the province wealthy.

The main industries in Yuhuan county, south of Taizhou , are fish farming and manufacturing.

On Thursday night, Mr Zhang returned with several others to demand justice. The group threw stones at police officers, damaged police vehicles and obstructed officers from duties, the Yuhuan government said.

Protesters damaged six police motorcycles, causing fuel to leak out. Three officers were injured. Authorities detained 23 people.

On Friday night, more workers came, surrounded the station and demanded that the people be released, but police dispersed them.

On Saturday, the number of protesting workers started in the dozens but swelled to more than 1,000. They surrounded the police station, throwing rocks to smash windows and damage other property. Once again authorities cleared the crowd.

More than 300 People's Armed Police, a paramilitary force, were patrolling the area. Local officials said only one person had been arrested and the rest had been released on bail, but the human rights group disputed that claim.

The government defended its handling of the incident, saying it was necessary to 'maintain social stability'. It said the situation was now calm.

Local officials took a tough stance at a news conference on Sunday. They acknowledged migrant workers had made contributions to the economy but said the new residents - estimated in the thousands or even tens of thousands - had put pressure on societal management.

'We will investigate to the end, will strike severely and will not be soft-hearted,' said Weng Zhengui , the county's deputy police chief. Officials denied rumours that people had been killed in the clashes.

On June 28, tens of thousands of residents of Wengan in Guizhou province stormed government and police offices over what they believed to be a cover-up of the rape and murder of a teenage girl, allegedly by the son of a local official.

The government said the girl committed suicide and blamed the protests on public dissatisfaction caused by crime and corruption.