BEIJING Migrant workers in eastern China rioted and protested for three days last week, officials said, vowing tough steps to quell the latest ripple of unrest ahead of the Beijing Olympics.

The clashes broke out in Yuhuan County in coastal Zhejiang province, where dizzying manufacturing growth has attracted a torrent of migrant workers from poor parts of the countryside.

The county government gave a sparse account of the confrontation on a local official Web site (yhnews.zjol.com.cn). But officials left no doubt they were alarmed.

"This was a grave crime of obstructing public security organs in carrying out their duties and assembling a crowd to attack state offices," a deputy head of the Yuhuan police force, Weng Zhengui, told local reporters, according to the Web site.

"We will thoroughly investigate it, sternly attack and show no softness."

Thousands of protests, riots and "mass incidents" occur in China every year, most of them small and never openly reported.

But the recent ripple of reported unrest comes at a tense time,

with Beijing readying for the Games in August that it has

promoted as a show of social progress and harmony.

Rioting broke out in Kanmen town in Yuhuan on the night of July 10 after a migrant worker surnamed Zhang came to a local law-and-order office to complain about injuries from "colliding with a wall" there the previous day, said the account posted late on Sunday. The report did not say how he collided with the wall.

Zhang agreed to be taken to a hospital by police, but on the way an angry crowd surrounded the police and yanked the fuel pipes out of six of their motorcycles.

"Seeing that the situation was tense, the police decided to first withdraw, but while they were doing so the surrounding crowd threw rocks at them, injuring three police officers."

The report did not say how many rioters attacked the police, but did note that 23 people were "dealt with according to the law" after authorities regained control.

The Hong Kong-based Information Center for Human Rights and Democracy, a group that often reports on unrest in China, said 300 anti-riot police were deployed to restore control.

Calls to village and county offices were not answered.

But on the following night, angry migrant workers again assembled in front of the Kanmen police station to "stir trouble," the report said, and again on July 12 a crowd surrounded the station and smashed some windows.

Weeks before the Beijing Olympics, Chinese officials have been told to keep a tight lid on any flickers of unrest.

But a riot involving 30,000 in the southwest Chinese town of Wengan Fugu County, Shaanxi province has prompted widespread media discussion about the sources of continued discontent.

Officials in Yuhuan said they would issue rules to strengthen management of migrant workers while also opening a "rights defence centre" for them.