Riot police used tear gas in an attempt to disperse the protesters at the highway in the town of Haimen in Guangdong province, and the demonstrators hurled rocks, water bottles and bricks in return, said one of the protesters, a 27-year-old man surnamed Chen.
It is the second major protest in two weeks in a corner of coastal southern China that has been seeing periodic unrest over the last few years, primarily over land disputes. In much of Guangdong province, conflicts have been intense because the area is among China's most economically developed, pushing up land prices.
In Wukan, a village to the southwest of Haimen, protesters drove local authorities from the area nearly two weeks ago over a land dispute. Wukan protesters reached by phone Tuesday said plans for a large march on a nearby government office on Wednesday would go ahead.
In Haimen, some protesters clashed with police, leaving dozens hurt including women and police. Some in the crowd speculated that one man who was lying on the ground bleeding from his head had died, but that could not be confirmed, Chen said.
"We don't have any weapons, only mineral water bottles and we threw them at the police but the police were using batons to beat people up," Chen said in a phone interview. He estimated that around 20,000 people participated in the demonstration.
The protesters had first gathered in the morning at the office of the Haimen government and demanded a meeting with the township party secretary, Chen said.
A woman who answered the phone at the Haimen government office said the protesters had left and then hung up.
The protesters think that an existing coal-fired power plant has contributed to what they say is a spike in cancer cases and heavy pollution in the seas, a serious problem for a town where fishing is a source of livelihood.
"People are worried about the pollution that will be released by the (new) power plant," said Wang Xiebo, a fisherman reached by phone.
Another protester, a man surnamed Yang, provided a similar account of the protest and subsequent clash.
"Two or three of us fainted on the ground when they fired tear gas at us," Yang said. "The government offended us again and again by trying to build a power plant. This is going to affect our future generations. They still need to live," Yang said.
Photos circulating on China's popular Twitter-like microblog Weibo showed a crowd of protesters amassed at a large government building and then at a highway, as well as riot police with plastic shields and helmets lined up in tight rows. Some photos showed protesters and police injured and bleeding.
After three decades of laxly regulated industrialization, China is seeing a surge in protests over such environmental worries.
In September, hundreds of villagers in an eastern Chinese city near Shanghai demonstrated against pollution they blamed on a solar panel factory. In August, 12,000 residents in the northeastern port city of Dalian protested against a chemical plant after waves from a tropical storm broke a dike guarding the plant and raised fears that flood waters could release toxic chemicals.
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