China's security chief has urged authorities to resolve conflicts swiftly and enforce the law in a "civilised" way after an upsurge in violent protests in the southern province of Guangdong.

The comments by Zhou Yongkang, a member of China's top ruling body the Politburo Standing Committee, underscore the Communist Party's growing concern over unrest ahead of a generational leadership transition starting next year.

They came after two days of violent clashes between police and residents of a Guangdong town who are protesting against the expansion of a coal-fired power plant they say causes harmful pollution.

Witnesses in the town of Haimen have described police beating protesters and firing tear gas, and said a 15-year-old boy and a middle-aged woman were killed in the clashes, although AFP has not independently confirmed this.

The official Xinhua news agency on Thursday quoted a local Communist Party official denying anyone had died.

The riots, which began on Tuesday but appeared to have calmed Thursday, came in the same week residents of Wukan village agreed to end their long stand-off with authorities after a senior Guangdong official intervened.

Angered by years of illegal land grabs, the villagers had driven out local Communist party officials and elected their own leaders, in a rare revolt that attracted international media attention.

But they called off the protest this week after a senior official said their complaints about land seizures were "reasonable" and agreed to release three detained protest leaders.

Authorities freed one of the leaders on Thursday, his brother told AFP, with the other villagers and the body of a fourth, who died in police custody, to be released in the coming days.

Zhang Jiancheng "came back home this afternoon. He told me 'I've been released on bail so I can't tell much and I can't accept interviews'," Zhang Jianxin said.

"He's much thinner than before... he was a very active person before, but now he looks quite dull."

Beijing sees social unrest as a major potential threat to its rule, and on Wednesday Zhou urged local authorities to resolve disputes before they could get out of control.

"(We must) deepen our efforts to mediate conflicts and disputes, improving the system of mediation to resolve conflicts and disputes at the grassroots level and nip them in the bud," said Zhou, in comments reported by the official Xinhua news agency.

"We must also adhere to civilised standards of law enforcement, and deal with mass incidents and individual extremist events according to the law."

He did not specifically mention the handover of power to a new generation, but said authorities should be especially vigilant against unrest ahead of 2012, which he called a year of "special significance" to China's development.

"Every political and legal organ must enhance their political awareness... and their sense of responsibility," he said.

"We must take the initiative and effectively prevent and crack down on all separatist and disruptive, violent and terrorist criminal activities."

The latest protests in Guangdong, China's wealthiest province and its manufacturing heartland, follow months of labour unrest as the country's export growth slows, forcing factory owners to cut pay and reduce staff.

The unrest has piled pressure on the province's reformist Communist party secretary, Wang Yang, who is tipped to win promotion to the Politburo Standing Committee next year.

But on Thursday the People's Daily, the mouthpiece of the ruling Communist party, heaped praise on the efforts of the provincial authorities to defuse the protests in Wukan.

The acknowledgement by Guangdong officials that most of the people's demands were reasonable had helped to resolve the crisis, the paper said, while the Global Times warned that such protests were likely to increase.

"Putting the public first and helping them fulfil reasonable interests should be the aim of local government officials," the newspaper said.