Police in a south China city bordering Vietnam clashed with thousands of rioters who were protesting excessive brutality meted out to a suspected smuggler, the government and a rights group said.

The incident occurred Friday in Dongxing city in Guangxi Zhuang Autonomous Region, with rioters destroying numerous police and border patrol vehicles and clashing with security forces, local police said in microblog postings.

"Five policemen were lightly injured and nine anti-smuggling vehicles were damaged... no one (else) at the scene was injured or killed," the Fangchenggang prefecture public security bureau, which oversees Dongxing, said on its microblog site late Friday.

"During the incident the police did not adopt overly violent behaviour."

According to the Hong Kong-based Information Centre for Human Rights and Democracy, up to 10,000 people joined in the rioting and over 20 police and customs vehicles were smashed or burned.

Up to 100 people were "killed or injured" during clashes, it said.

Increasing numbers of anti-riot police and security forces arrived in Dongxing throughout Friday.

The rioting erupted after locals on a busy Dongxing street saw a motorcyclist being treated brusquely by anti-smuggling police after crashing their vehicle into him, authorities and the centre said.

"Some people in the crowd thought the motorcyclist was dead and they refused to let the police vehicles leave... they began overturning and torching police cars," officials said.

Police estimated around 1,000 people were at the scene of the riot, but only a handful of people engaged in lawless behaviour.

Photos posted online showed a line of overturned police and government vehicles, some on fire, and a tense line of riot police confronting crowds.

Calls to Dongxing government and police went unanswered Saturday.

Academics estimate China saw 180,000 protests last year over a wide range of issues including corruption, government-backed land grabs, police brutality and unpaid wages.