It was the latest in a string of protests in China this year over fears of health risks from industrial projects, as members of the rising middle class become more outspoken against environmentally risky projects in their areas.
Past protests have targeted a coal-fired power plant in southern China, a waste-water pipeline in eastern China, and a copper plant in west-central China.
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 is under evaluation and the public has opportunities to offer its input.
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.
He said demonstrators overturned a car, and some smashed the door of a fire truck that arrived to hose off leaked gasoline as well as to disperse protesters.
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 form 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, people gathered in a large public square in Ningbo and shouted slogans against the project, Ren said. Many were taken away by police, he said.
Among the protesters was a businessman who said he was taken to a police station where he was forced to delete photos of the protest he had taken with his cellphone. He said he was released after an hour.
"The police have my information, and I need to keep a low profile," said the man, who gave only his family name, Wang.
The man, who was reached by phone, estimated there were tens of thousands of protesters on the square and that some were beaten by police.
Another man who gave the surname Mao said he happened to be in the square and began taking photos when he saw police were throwing people, including women and the elderly, onto the ground and severely beating them.
"I wanted to document it," Mao said.
Mao said he was forced into a police vehicle and detained in a police station for more than eight hours before being released with a warning that he should not post material online or accept interviews.
Photos posted online showed demonstrators clashing with police and holding up signs demanding that the project be halted. Some were shown being dragged away by police.
Searches for phrases including Zhenhai and Zhenhai chemical plant were blocked Saturday on China's extremely popular microblogging site Sina Weibo.
But that did not stop people from sharing information and expressing opinions online.
A microblogger in the town of Cixi, 50 kilometers (30 miles) away, said the project set off anger in an area where residents are prone to cancer because of severe pollution caused by industrial development.
"Many of my family members, friends and neighbors have cancer," said the microblogger, who gave only his last name, Hu, citing the sensitivity of the matter. "Our area is economically developed enough that we don't need projects that harm people's health."
"Government officials are only concerned about GDP and climbing up their career ladders, which is incompatible with local residents' desire to have a pleasant life," he said. "The officials will move on after a few years, but we will be living here generation after generation. It is irresponsible to build this project."
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