QIDONG, China — More than 5,000 Chinese protesters staged a demonstration Saturday in Qidong, Jiangsu Province, against the planned construction of a pipeline to discharge waste from an Oji Paper Co. plant into the East China Sea.

More than 1,000 of those at the event, which was organized over the Internet, broke into the offices of the local government. Some threw documents from windows and smashed windowpanes at its entrance. At least 10 vehicles, including a police car, were vandalized.

Some demonstrators jeered at top local officials for “selling out the country,” while others voiced criticism of Japan over the disputed Senkaku Islands. Numerous messages on Chinese websites linked the pipe issue with the inflammatory territorial row.

Major anti-Japanese demonstrations were staged in various parts of China following a September 2010 clash near the Senkakus between a Chinese fishing boat and Japan Coast Guard cutters. The police did not move to control them, in apparent approval of such protests.

Observers said the authorities may have allowed Saturday’s demonstration in the regional city to proceed with the aim of pressuring Japan to abandon its plan to nationalize the Senkaku islets.

The local Qidong government pledged to the demonstrators that it will withdraw the drainage plan and called on them to disperse. Protests against environmental degradation have increased recently in China, where three decades of rapid and unfettered industrial expansion have taken a heavy toll.

Oji Paper, meanwhile, denied that its paper mill is polluting the environment.

“We don’t release ‘polluted water,’ as we are currently releasing water after purification that meets local environmental standards,” a company spokesman was quoted as saying.

The plan calls for building a pipeline to carry waste from the plant in the city of Nantong roughly 100 km to the coast in Qidong. The discharges would reach 150,000 tons a day when the mill becomes fully operational, according to residents quoted Friday by the state-run Global Times newspaper.

Construction of the mill started in 2007. The Qidong authorities announced Thursday that work would be suspended, but the demonstration went ahead anyway to demand the project be scrapped altogether.

Qidong is located about 80 km north of central Shanghai, while a number of Japanese-affiliated companies do business in Nantong.