Tens of thousands of protesters marched through the streets of Dalian on Sunday to demand the relocation of a chemical plant that has been constructed on the coast of the port city in northeast China.

The demonstration - one of the biggest in a series of recent NIMBY rallies against potential polluters in China - was sparked by the news last week that a protective dike around the Fujia factory in the Jinzhou industrial complex had been breached by rain and high waves ahead of the approach of Typhoon Muifa.

Minor scuffles were reported, but the ranks of riot police mostly looked on as largely peaceful and white-collar crowd - variously estimated at 10,000 to 70,000 - assembled without a permit in People's Square.

Some chanted old revolutionary songs. Others held banners that spelled out local concerns about the factory, which produces paraxylene (PX) - a benzene-based chemical widely used in plastic bottles and polyester clothing.

Mobile phone photographs of the protest have circulated rapidly on the Chinese internet. The slogans included "PX Out!", "Refuse PX", "PX out of Dalian". Some protestors wore gas masks. Others stood on top of a police car.

The city's Communist Party chief Tang Jun and mayor Li Wancai attempted to mollify the crowd with a promise to move the polluting project out of the city," according to the Xinhua news agency. Participants said they were heckled with calls of "Get out!" and "We want action."

The gathering was organised through microblogs and social networks such as Sina Weibo and Renren, despite the efforts of censors to remove comments calling for action. Witnesses said local people gave free water to the protestors, many of whom had prepared T-shirts and posters.

David Gao, a local student, said the demonstration grew so large that it expanded beyond the massive concrete square outside the city government office and into local streets.

"We need to kick the PX factory out of Dalian to protect our environment. If something as serious as the storm happens again, it will be a disaster," he said.

Although poisons are not thought to have leaked during last week's storm, thousands of local residents were evacuated. Others said they did not trust official reports.

"Even if there was contamination, the government would restrict the news," said Cindy Xin, who was taking part in a demonstration for the first time.

Concerns about the safety of the plant were heightened by reports that Chinese reporters who went to investigate were denied access and beaten by security guards.

This incident put the spotlight back on paraxylene (PX) - a benzene-based chemical widely used in plastic bottles and polyester clothing. PX plants are far from being the most dangerous of China's chemical facilities, but they have proved politically toxic.

In 2007, tens of thousands of residents in the southern city of Xiamen went on a protest walk against a PX plant. This prompted the local government to move the facility out of their jurisdiction.

Several of the online notices about the Dalian protests referred to the Xiamen case, the large oil spill in Dalian last year and recent leaks from an oil field in nearby Bohai Sea that went unreported for a month.

Additional research by Han Cheng