Authorities in east China ordered a security crackdown Sunday after thousands of locals clashed with police during a protest over the construction of a chemical plant.

In the latest environmental unrest to erupt in China, police in Ningbo city, Zhejiang province, fired tear gas Saturday night after six days of demonstrations over the project, online reports said.

"In recent days some unreasonable activities such as illegal gatherings and rioting have occurred, seriously impairing the normal work and life of the people and severely impacting overall development and stability," a statement on the website of Ningbo's Zhenhai district government said.

In an emergency meeting late Saturday local party officials insisted that the chemical plant project had not been formally approved and agreed to listen to the demands of the protesters, the statement said.

The government also ordered police to "maintain stability in accordance with law", rhetoric that often signals a heavy-handed crackdown by the authorities.

Photos posted on the www.molihua.org website, which monitors social unrest in China, showed protesters facing off against thousands of riot police as security forces streamed into Ningbo's Zhenhai district where the 55.9 billion yuan ($8.9 billion) plant is to be located.

Rioting erupted amid rumours that police had beaten to death a local college student -- a rumour police immediately denied.

"Those people circulating fabricated rumours that 'police have beaten to death a college student' have had an odious social impact," Zhenhai police said in a posting on the force's microblog site.

"Following investigation it was found that a certain woman had spread the rumour, (she) will be dealt with in accordance with law. The police warn citizens not to believe rumours and do not spread them."

Government officials and police in Zhenhai refused to comment on the unrest when contacted by AFP, nor would they say if protests were continuing Sunday.

The demonstration began on Monday last week when some 200 residents in Zhenhai blocked traffic before dispersing, the local government and state media said at the time.

Environmental pollution and perceived health threats are sparking protests across China, fuelled by social media which allows organisers to publicise their causes and rally others despite tight controls in the one-party state.