Rioters in the southern Chinese province of Guangdong have besieged government buildings, attacked police officers and overturned SWAT team vehicles during protests this week against the seizure of farmland, said officials in Shanwei, a city not far from Hong Kong that skirts the South China Sea.

According to a government Web site, hundreds of people on Wednesday blocked an important highway while others mobbed the local headquarters of the Communist Party and a police station in the city of Lufeng, injuring a dozen officers. Some witnesses, posting anonymous accounts online, put the number of rioters at more than one thousand.

The protests continued Friday, with farmers gathered in front of a government building banging gongs and holding aloft signs that said “give us back our farmland” and “let us continue farming,” Reuters reported.

The authorities say the violence escalated Thursday after rumors spread that the police had killed a girl. At least four people were arrested, including a man officials accused of instigating the crowd.

The violence was the latest outbreak of civil unrest in China fueled by popular discontent over industrial pollution, police misconduct or illegal land grabs that leave peasants with little or no compensation. Such “mass incidents,” as the government calls them, have been steadily increasing in recent years, providing party leaders with worrisome proof that official malfeasance combined with a dysfunctional judiciary often has combustible results.

According to a recent study by two scholars at Nankai University in Tianjin, there were 90,000 such episodes in 2009, a figure that includes melees and mass petition campaigns by people seeking justice. Government censors often work hard to make sure such events stay off the Internet and out of newspapers.

Last week, hundreds of residents protesting environmental contamination by a solar panel factory in Zhejiang Province stormed the factory and destroyed office equipment and vehicles. Weeks earlier, 12,000 people peacefully gathered in the city of Dalian to demand the closure of a chemical factory.

In Lufeng, the protests were just the most dramatic manifestation of a long-running battle over land that residents say their ancestors reclaimed from the sea. According to the bulletin board of a local Web site, the Lufeng city government has already sold off more than 800 acres, of the property for industrial parks and high-priced housing. The proffered compensation per acre, villagers said, has been barely enough to buy a new bed.

“Wake up, my neighbors, if we don’t unite now, the land of our ancestors will be sold off to the last square meter! If we don’t unite now, our children will be homeless!” read one posting on the site.

“We will have no where to bury our parents or raise our children!”

Municipal governments, which own all land in China, largely depend on sales of long-term property leases to fill their operating budgets. In many cases, private real estate companies collude with officials to clear and develop the land as quickly as possible.

The latest seized plots were sold to a developer for 1 billion renminbi, or about $156 million, according to The South China Morning Post, which first reported the sale and seizure. According to the company’s Web site, the complex is to be called “Country Garden” after the name of the developer.

“To shape a prosperous future through our conscience and social responsibility,” is one of the company’s mottos.

News of the demonstrations as well as photos and videos were quickly deleted by censors, but a few images persisted Friday. One showed demonstrators carrying a banner that read “Give back my ancestors’ farmland.” A video lingered on overturned police vehicles, including one with graffiti that read “running dogs,” an insult once directed at perceived enemies of the people.

The continuing unrest could pose a threat to the political aspirations of Wang Yang, the provincial party secretary who has partly staked his reputation on promoting the well-being of Guangdong’s 86 million residents and by trying gauge the level of their happiness.

“Happiness for the people is like flowers,” Mr. Wang wrote this year in a state-run publication. “The party and the government shall create the proper environment for the flowers to grow.”

The province is China’s most populous and a manufacturing powerhouse that produces roughly one third of the country’s exports.