Thousands of protesters took to the streets of the southwestern Chinese city of Luzhou on Wednesday after reports a truck driver had been beaten to death by policemen, residents said, but state media said the driver had died after falling ill.
Pictures and video on China's popular microblogging site Sina Weibo showed a body sprawled out on the ground next to a truck as police held back onlookers.
Reuters was unable to independently verify the photos and calls seeking comment from the Luzhou government in Sichuan province went unanswered.
China's Communist Party has been trying to keep a lid on protests ahead of a meeting in Beijing next month which will usher in a new generation of leaders.
Residents contacted by telephone said they had heard reports that traffic policemen had beaten a truck driver to death after an unspecified dispute.
"People are very angry about this and are out on the streets to show their anger," said one resident of the Hongxingcun neighbourhood where the unrest was focused. He did not witness the incident and declined to give his name.
A manager at a local restaurant who gave her family name as Wang added that several thousand people had taken to the streets.
Images posted later in the evening showed overturned police cars, some of which had been set alight. Some Weibo posts said police had used tear gas to disperse the demonstrators.
State media said on Thursday that five police cars had been set on fire and 20 people detained. But it denied the driver had been beaten to death.
The official Xinhua news agency said the driver had "suddenly felt uncomfortable" during a scuffle with police about where to park his truck, and doctors called to the scene were unable to save his life.
"As a result, some people attacked police cars at the site, the report added.
China's ruling Communist Party worries that the tens of thousands of sporadic protests over land grabs, corruption, abuse of power and economic grievances that break out every year could coalesce into a national movement and threaten its control.
China saw almost 90,000 such "mass incidents" of riots, protests, mass petitions and other acts of unrest in 2009, according to a 2011 study by two scholars from Nankai University in north China. Some estimates go even higher.
That is an increase from 2007, when China had over 80,000 mass incidents, according to an earlier report from the Chinese Academy of Social Sciences.
Most protests are either dispersed by security forces, or by officials promising demonstrators their demands will be heeded. None have so far even come close to becoming national movements which could challenge the central government.