NANTONG, China—Hundreds of demonstrators ransacked a local government building on July 28 in this coastal city in protest at plans by a Japanese company to pump industrial waste water into the nearby sea.

About 10,000 residents from the city’s Qidong district along the Yellow Sea had gathered to protest the dangers posed by waste water from a local paper mill operated by Oji Paper Co.

The demonstrators assembled at 6 a.m. and began chanting “Boycott Oji (products)!” Ignoring police warnings, hundreds then swarmed the local government building. Protesters broke down the building’s steel gate and shattered glass doors. Once inside, they turned over desks, computers, papers, bottles of liquor and other items in a number of office rooms.

The plant is located at an industrial complex in the city about 100 kilometers inland. The city government had promised to build a drainage pipeline to discharge waste water into the sea when it attracted Oji Paper and other companies to the complex, and construction of the pipeline was under way.

But fears began to spread among local residents that the discharged water contained carcinogens and other contaminants.

Senior local officials announced that they will cancel construction of the drainage pipeline.

The plant went into operation in January 2011 and currently discharges waste water into Changjiang River.

“We have not received any direct complaints,” said an official with Oji Paper. “The waste water is treated and we are not aware of any pollution.”

The official said the riot had not affected the plant’s operation.