"The police accused them of using the Internet and other means to spread rumours, inciting trouble or illegally marching or demonstrating, or using the Internet to spread rumours and harmful information," the report said.

About 200 people took to the streets last week to demonstrate against plans for the ethylene plant and oil refinery in Chengdu's northern outskirts, an echo of a protest movement that forced the government to scrap plans for a chemical plant in the southern city of Xiamen.

In March, officials in Xiamen confirmed they would shift a proposed plant to make paraxylene, a petrochemical used in polyester and fabrics, after thousands took to the streets and forced a rare invitation from the government for public comment.

China's Communist authorities frown on public protest, but demonstrations are becoming more common due to anger over official corruption and pollution and tensions between industrialisation and environmental concerns.

The Chengdu protesters, who news reports said were orderly and did not carry banners, worried the plant would lead to degradation of air and water quality.

The ethylene plant was due to produce 800,000 tonnes a year of the industrial compound commonly used in packaging and insulation.

The refinery, which would process 10 million tonnes of crude oil a year, had been approved by China's top planning agency, the National Development and Reform Commission, last year, the Beijing News earlier reported.