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BEIJING, May 31 (Reuters) - Hundreds of Chinese peasants have protested over family-planning rules in an area hit by similar demonstrations earlier this month, state media said on Thursday, with some setting fire to government vehicles.

The protesters in Yangmei township in the southwestern region of Guangxi gathered in front of a family-planning office on Tuesday to demand a refund of fines levied for having more than one child, the China Daily said.

"Some of them forced their way into the office, smashed furniture and set fire to government vehicles," an unnamed local government spokesman was quoted as saying.

China launched its one-child policy in 1980 to curb a ballooning population, now at more than 1.3 billion. The rules vary from city to countryside but usually limit families to one or, at most, two children.

The sometimes arbitrary enforcing measures, such as hefty fines and forced abortions, have long fuelled tension between officials and residents.

Social unrest has also been fuelled by corruption, illegal land grabs and a rising gap between rich and poor, challenging the ruling Communist Party's efforts to maintain stability and its grip on power.

Thousands of people rioted in several towns in the nearby county of Bobai in Guangxi from May 17-20 in possibly the most drastic outbursts of discontent over the drive so far, ransacking government buildings, burning cars and clashing with police.

In the latest protests in Rongxian county, residents brandished "fake" government papers claiming that the penalty for a second child had been reduced to 1,000 yuan ($130), while they had been forced to pay at least 10,000 yuan, the China Daily said.

A similar incident was reported in Lingshan township in the same county on Tuesday when several hundred people smashed windows at the township government office, the newspaper said.

Local officials had defended the fines as "in line with laws" and calm had returned to the two townships later in the day, the newspaper said.

Seven people have been detained for instigating the riots, Yulin city government, which administers Bobai and Rongxian, said in a report on its Web site (www.yulin.gov.cn) on Thursday.

In a televised speech aired on Wednesday, the police chief in Yulin vowed to crack down on the "law breakers" and called on residents to inform on their whereabouts.

"Recently, law breakers have organised and instigated the unwitting masses to stir up trouble, besiege government buildings and spread forged official family-planning documents," Ding Shan was quoted as saying in the speech on the Yulin Web site.

"Family planning is a basic national policy which should be strictly enforced. Our city has carried out the policy according to law."

Chinese officials credit family-planning rules with keeping the country's current population to about 1.3 billion and so boosting prosperity.

But the policy is resented in many parts of the restive countryside where children, especially boys, are considered a safety net. ($1 = 7.649 yuan)

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