A city in China's southwest has scrapped plans for a copper alloy plant after thousands of people protested over possible health risks.

Thousands of people protested?possible health risks?over a plan of building a copper alloy plant in?Shifang, Sichuan?Province.

The government of Shifang City in Sichuan Province issued an online statement yesterday announcing its decision to halt the molybdenum-copper alloy project.

The project was the city's first one with an investment exceeding 10 billion yuan (US$1.6 billion), officials said. The project by Shanghai-listed Sichuan Hongda was expected to bring in more than 50 billion yuan in annual sales, which would mean tax of about 4 billion yuan.

On Monday the city government said it would delay the project after the protest and educate residents about it.

The city's public security bureau had earlier warned the public not to use the Internet or cellphones to organize more riots and asked those who had done so to turn themselves in within three days or face severe punishment.

Thousands of people - including high school students - began to gather in front of the city government building and a public square on Sunday night, and violence erupted on Monday.

Protesters broke through a police cordon and stormed into the government office complex on Monday morning, according to Shifang's publicity department. They shattered windows and bulletin boards.

At 1:30pm more residents gathered at the office complex and threw flowerpots, water bottles and other objects at police and government workers.

Many officers were injured and nearly 10 vehicles were damaged during the riot.

Then the protesters pushed down the gates of the complex, the government statement said. Officers used tear gas and batons to disperse the crowd at about 2pm. The statement said 13 protesters were taken to hospital.

Public anger surged as Internet users circulated photos and videos of the riot police in action. Some Internet users said one protester had died.

"People are very upset. How could the police beat them?" a 15-year-old middle school student surnamed Liu, who did not join the protest, told The Associated Press.

A man who answered the phone at Shifang No. 2 Hospital said more than 30 people, including police officers and protesters, had been treated for minor injuries and discharged. The man, who declined to be named, said no one had died.

Liu said parents, classmates and teachers all objected to the project because of its environmental risks.

"It will make our home city a town of death," Liu said.

Shifang, with a population of around 430,000, is about 50 kilometers from Chengdu, the provincial capital. Nearly 6,000 people in Shifang lost their lives in the massive 2008 earthquake that hit Sichuan and neighboring areas.

Pollution problems are a leading cause of public anger in China as the country undergoes rapid economic development.

In recent years, people have become more outspoken against environmentally risky projects in their backyards.

Last year, a chemical factory was moved from Dalian in northeastern China when 12,000 people protested.