By Francis Markus
BBC News, Shanghai
Authorities in China have been tolerating some protests
About 100 police officers and officials are reported to have been injured during an anti-pollution protest in eastern China, hospital sources say.
A riot erupted after demonstrators attempted to block a shipment of goods from leaving a chemical factory in Zhejiang province.
The protest is one of several outbreaks of rural unrest in recent months.
Details of the rioting which broke out on Sunday are still sketchy.
A local resident said the incident flared up when protestors who had been encamped outside a chemical factory for about two weeks, tried to stop a shipment of goods from leaving the plant.
Violent protests against the police started after two elderly women protestors were killed in the police action, reports news agency Reuters.
It is not clear how they died.
The protestors have been campaigning against the pollution they complain that local chemical factories are causing.
It coincided with a wave of anti-Japanese demonstrations in major Chinese cities.
Those protests are tolerated by the authorities, letting people vent their anger.
But some observers warn that they have at least the potential to spin out of control and combine with an array of domestic political grievances.