A Chinese man accused of participating in a riot over land claims in September died in police custody on Sunday, threatening to fan tensions in a far southern region that has become a source of persistent unrest.

The death in Guangdong province occurred as masses of riot police moved to quell a longstanding dispute in Wukan village on the east coast of the booming province, where industrial development has consumed swathes of rice paddies.

The government of Shanwei, an area that includes Wukan village in its jurisdiction, said in the early hours of Monday that Xue Jinbo fell ill on Sunday, his third day in detention over the riot. Hospital doctors later pronounced the man dead despite frantic efforts to save his life.

In an apparent effort to head off further trouble in the area, officials immediately notified Xue's family and offered aid, said the announcement on the Shanwei government website (www.shanwei.gov.cn).

The cause of death was cardiac failure, and other causes of death have been provisionally ruled out, said the government notice, citing hospital doctors.

The death in custody nonetheless threatens to stir fresh tensions.

Land grab disputes are an explosive social issue in rural China, with disputes spreading to the undeveloped west of the country, according to a recent poll published in Outlook Weekly, a magazine run by China's Xinhua News Agency.

Pictures on microblogging sites from Wukan village showed large numbers of riot police standing off with residents who are demanding the return of farmland to restore their livelihoods.

Hong Kong's Apple Daily newspaper and two villagers contacted by Reuters said police had used tear gas and blocked all roads to the village.

(Reporting by James Pomfret and Chris Buckley; Editing by Chris Lewis and Sanjeev Miglani)