Thousands of villagers from the Zhuang ethnic group clashed with Han workers over an aluminium plant that natives say dumped the sewage that poisoned their drinking water and flooded 100 homes.

Both Xinhua and local authorities in Jingxi county, in the Guangxi Zhuang Autonomous Region , said the clash was over a dispute concerning construction of a road to the plant.

Witnesses said several thousand angry Zhuang from nearby villages had surrounded the county government's headquarters on Tuesday afternoon after smashing equipment at the aluminium plant on Sunday.

Huang An , a Zhuang from Lingwan village, said the local government had mobilised more than 1,000 riot police to quell the protest. "The road leading to the county government building, which is several kilometres long, was packed with villagers holding slogans, and armed policemen fired into the air to warn the furious protesters," he said.

At least five of his fellow villagers had been wounded in the confrontation and the figure could rise once casualties from other villages were counted, he said. Photos taken by witnesses showed a large number of riot policemen had sealed off roads in the county and the atmosphere in the streets was tense. The protesters, who say the plant polluted the only source of drinking water for a dozen villages and caused flooding, had painted slogans on their clothes.

The Information Centre for Human Rights and Democracy said yesterday more than 100 people were injured in the riot and at least 10 vehicles, including a police car and an armoured vehicle, were smashed by angry villagers in the protest. It said the riot continued yesterday morning, with more protesters injured.

A Jingxi county propaganda official said one official had been injured in the riot after villagers had blocked roads and pelted policemen and government officials with stones. "The confrontation first broke out on Sunday afternoon, when the aluminium plant tried to reconstruct a road next to Lingwan {hellip} Villagers later protested outside the plant and smashed equipment in the evening," the county said in a statement. "A protest organised by villagers on Tuesday afternoon blocked the expressway and the county's road traffic."

The official claimed the riot had quietened down yesterday.

Zhuang from nearby villages complained pollution from the aluminium plant, located in the upper reaches of an underground river, had blocked their only water source in March during a construction project.

"Thousands of people were short of drinking water for several months, at a time when the autonomous region was having a once-in-a-century drought," a villager said. The drought this year left more than 2.2 million people and 1.1 million head of livestock short of water and 740,000 hectares of farmland too dry to plant.

"The plant was later forced to change the course of a river for villagers living in the lower reaches. But the water is red and heavily polluted by untreated industrial sewage discharged from the plant. We don't dare drink water from it," he said.

Meanwhile, residents of Lingwan village complained more than 400 people in the upper reaches of the river were affected by flooding for three months after the plant had blocked the same underground river.

"Authorities claimed the flood was caused by a slight earthquake, but we believe the plant sealed off the underground river by mistake after it had tried to demolish a mountain during a construction project," he said. The Guangxi Daily reported last month that the county had spent 7 million yuan (HK$8 million) over two months to drain off floodwater. He said the riots on Sunday and Tuesday were triggered by the plant's security guards, who beat villagers who had been protesting outside the plant.

More than 96 per cent of people in Jingxi county are Zhuang - who make up one of the five largest ethnic minorities in China. Last year, a fight between Uygur and Han workers at a factory in Shaoguan , Guangdong, left two Uygur workers dead and 118 injured. That fight triggered rioting by the two ethnic groups in Urumqi last July that left almost 200 people dead.