BEIJING (AP) -- Thousands of people in an eastern Chinese city clashed with police while protesting the proposed expansion of a chemical factory that they say would spew pollution and damage public health, townspeople said Saturday.

Pollution has become a major source of unrest in China, as members of the rising middle class become more outspoken against environmentally risky projects in their backyards.

The Zhenhai district government in Zhejiang province's Ningbo city said in a statement Saturday that a few people disrupted public order by staging sit-ins, unfurling banners, distributing fliers and obstructing roads. It said the proposed project was under review.

Zhenhai police said protesters threw rocks and bricks at officers Friday and that police dispersed illegal gatherings to restore the flow of traffic.

Residents, however, said the protests involved thousands of people and turned violent after authorities used tear gas to dispel the crowds and arrested participants.

"It started with a peaceful petition but turned into a citywide riot," said a local resident who gave only his family name, Ren, because he had come under police watch. He said he was called in by police over his frequent online postings about the project, which would produce chemicals such as ethylene and paraxylene.

Ren said the protest intensified Friday when young residents returned home for the weekend. He said 4,000 to 5,000 people blocked major road entrances to the district and that the public grew angry when police arrested three college students and used tear gas on the crowds.

Thousands of protesters stormed a local police station, where they demanded the release of the students and a dialogue with district officials. Ren said the protesters also went to a traffic police compound, where they overturned police vehicles and private cars.

He said riot police moved in and formed a shield to guard the traffic police compound and that protesters threw rocks and water bottles at the riot police.

Later Friday night, police began beating protesters and passers-by with batons, Ren said.

On Saturday, protesters gathered in a large public square in Ningbo and shouted slogans against the project, Ren said. Many were arrested, he said.

Photos posted online showed protesters clashing with police and holding up signs demanding that the project be halted.

Searches for phrases including Zhenhai and Zhenhai chemical plant were blocked Saturday on the microblogging site Sina Weibo.

The state-run Ningbo Daily newspaper echoed the district government's statement in an online editorial Saturday, reminding the public that the expansion project remains under evaluation and saying there are ample opportunities _ including public hearings _ for the public to voice opinions. It also condemned any illegal behavior.

Residents of several Chinese cities have taken to the streets this year to protest projects such as a coal-fired power plant in southern China, a waste-water pipeline in eastern China, and a copper plant in west-central China.