At least 10 protesters and 29 policemen were injured as they clashed at a rally against a plan to build a waste incinerator in the capital of Zhejiang Province, official media reported.
One protester was severely injured but was out of danger.
A policeman too was severely injured, state-run Xinhua news agency reported.
More than 30 cars were attacked and overturned.
The protesters set two police cars on fire and smashed another four. Thousands took to the streets to protest against the plant, the Xinhua report said.
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The highway linking Hangzhou and Huizhou in Anhui province was closed for an hour-and-a-half due to the protest in Zhongtai township in Yuhang district of Hangzhou city, where the plant is scheduled to be built.
The site selection of the incinerator plant was made public in April.
Residents near the site fear that it will be harmful to their health and add to pollution. About 700 policemen were deployed to maintain order on the highway.