Hundreds of people protested against a proposed chemical plant in southwest China on Saturday, state media said, while residents in another city accused authorities of preventing a similar protest.

More than 200 demonstrators gathered in the city of Kunming to protest plans for a factory which will produce paraxylene (PX), a toxic petrochemical used to make fabrics, China's official Xinhua news agency reported.

Around 1,000 people described as "onlookers" surrounded the protesters, some of whom wore face-masks and held banners, the report said, adding that police "dissuaded" a protester from displaying a banner.

Police also lined the streets of Chengdu, the capital of southwest China's Sichuan province, after locals planned to demonstrate over a nearby chemical plant, residents contacted online by AFP said.

"There were a lot of police outside government offices, public spaces and important crossroads in the city," one resident surnamed Liu said, adding that fliers posted around the city in recent days had called for a protest.

"The fliers said the chemical plant has a big impact on people's health," he said, not wanting to give a full name for fear of official reprisals. The government responded with notices calling on people not to demonstrate, Liu said.

Photos posted online showed ranks of police lining the city's streets. Local police on Saturday morning announced that they would be carrying out an earthquake protection drill, a claim dismissed by thousands of Internet users.

"It's about preventing the protest," one user of the popular social networking website Sina Weibo wrote in response to the police notice. "This is the most blatant lie in the history of Chengdu," added another.

Locals online said that the protest did not take place.

Chengdu was shaken last month by a 6.6 magnitude earthquake which struck Lushan county, about 160 kilometres (100 miles) away, killing around 200 people.

Schools and universities in the city were requested to hold extra classes on Saturday, in an apparent attempt to keep people from protesting, several online reports said.

China has seen a number of urban demonstrations against proposed chemical plants in recent years, in what analysts have identified as a rising trend of environmentally-motivated "not in my backyard" protests in China.

Local authorities in the coastal city of Xiamen cancelled plans for a PX plant after thousands took part in a protest in 2007. A huge protest in the northeastern city of Dalian in 2011 prompted authorities to announce a similar climbdown.

The eastern city of Ningbo last year announced the withdrawal of plans for a PX plant after a demonstration involving about 200 people, while a violent protest in the southwestern city of Shifang prompted officials to shelve proposals for a metals factory.

Searches for "Chengdu PX" were blocked on Sina Weibo on Saturday, while posts about the Kunming protest were deleted by online censors. Local police in Chengdu declined to comment when contacted by AFP.