China on Friday blocked online access to news of riots by thousands of people who clashed with police in an eastern manufacturing city in what began as a protest over taxes.

Cars were smashed and several people were injured in the riots, which went on for more than a day and involved thousands of people, said local authorities on the website of eastern Zhejiang province, where the unrest occurred.

News and images of riots are viewed as highly sensitive in China and authorities moved swiftly to prevent users of the country's hugely popular social media sites from viewing online reports.

Searches for Zhili, the name of the town where the rioting took place, were blocked, as were words including "tax" and "protest".

The riots, which began on Wednesday and continued well into Thursday, were among the largest reported in China in recent months.

Hong Kong television showed footage from late Thursday of armed police on streets lined with shops whose windows had been smashed.

A local government website said 28 people had been arrested over the riots and police had been forced to use "heavy-handed measures" to quell them.

A posting on a microblog run by local police called for Internet users to "pay attention to the authenticity" of web messages about the incident.

Beijing has repeatedly vowed to clamp down on "rumours" on the Internet as the country's online population continues to grow.

China now has more than 500 million Internet users, posing a huge challenge to government attempts to control public opinion.

Authorities in Zhejiang said the tax collector whose demands for money from a clothing manufacturer sparked the Wednesday night riots had been sacked.

One local manufacturer reached by telephone said conditions were calm on Friday, but there were large numbers of police in the city and residents had been told to stay indoors.

Mass protests are not uncommon in China as disenfranchised people left behind by the country's economic boom take to the streets to air their grievances.

Last month, protesters in Zhejiang broke into a factory and ransacked offices, overturning vehicles after an Internet posting blamed the plant for local pollution.