Protests Against Expansion of China Chemical Plant Turn Violent Carlos Barria/Reuters The protesters, who witnesses said numbered in the thousands, were opposing the expansion of a state-run Sinopec plant, which is already one of the nation's largest refineries. BEIJING - A week of protests against the planned expansion of a petrochemical plant in the port city of Ningbo turned violent on Friday and Saturday when demonstrators attacked police cars and tossed bricks and water bottles at officers, according to accounts from participants posted on the Internet. Local residents, citing environmental concerns, have been demanding that the government move the plant from Ningbo, a prosperous city of 3.4 million in Zhejiang Province, not far from Shanghai. The clashes come at a delicate time for the government, as it prepares for a once-a-decade change in leadership that is scheduled to begin on Nov. 8 during a weeklong series of meetings in Beijing. Public concerns about industrial pollution have become a problem for the governing Communist Party, which often backs economic growth over public concerns about environmental degradation. In recent years, educated urbanites have harnessed social media to stage street protests against the construction or expansion of factories, mines and refineries. Although such demonstrations are illegal and organizers face arrest, they sometimes have the desired effect. In July, officials in Shifang, a city in China's southwest Sichuan Province, canceled plans for a huge copper smelter after tens of thousands of residents joined protests that turned violent. In September 2011, a solar energy company in Jiaxing, near Shanghai, was closed after demonstrators cited noxious chemicals used in the manufacturing process. And in August, officials in Dalian, in northeastern China, said a petrochemical plant would be closed and relocated after at least 12,000 people took to the streets. In a statement, the Zhenhai district government condemned those it blamed for organizing sit-ins and blocking roads in Ningbo but insisted that public sentiment would be taken into consideration before the start of construction. "Detailed information will be published when environmental reviews are implemented, and public opinions on the project will be heeded," the statement said. Residents have expressed concern about the refinery's production of ethylene and paraxylene, known as PX, a toxic petrochemical used in plastics, paints and cleaning solvents. The demonstrations, which began on Monday when 200 farmers blocked a road near the district government's office, according to the state media, grew larger on Friday, reportedly after student organizers issued calls through social media outlets. Photographs of the weekend demonstrations, many taken by cellphone, appeared to show riot police officers swinging batons as they chased protesters or beat those who had fallen to the ground. Censors worked quickly to delete images and witnesses" accounts posted on Sina Weibo, China's popular microblogging service. The Information Center for Human Rights and Democracy, an organization based in Hong Kong, said 10 people were injured after the police fired tear gas and moved to break up the protests, which took place in Tianyi Square in downtown Ningbo. In a series of online posts on Saturday, Chen Yaojun, a local lawyer, described how the police had quickly tackled and dragged away protesters who dared to chant slogans. He said he, too, was arrested after he tried to protect a young student who was being beaten by the police. After he was dragged into a police van, Mr. Chen said, he talked to a young policeman who expressed regret for the rough handling of the protesters. "We have no choice," the officer told him.