One hundred and forty people have been killed and more than 800 wounded in riots that rocked the western China at the weekend, the deadliest social unrest since the Tiananmen Square crackdown.

Running battles raged through the city of Urumqi throughout Sunday, pitting members of the Uighur minority against ethnic Han Chinese. Witnesses said that up to 3,000 rioters went on the rampage, smashing buses and overturning police barricades during several hours of violence.

State television showed cars in flames in the streets, and others being over-turned by rioters. Other footage showed a number of men attacking a man, apparently a Han Chinese, who lay on the street bleeding from the head and from injuries to other parts of his body.

Burnt-out buses lay scattered on the streets of Urumqi, the capital of China's restive, westernmost region of Xinjiang.

The death toll from the day of violence was put at 140 by the Xinjiang police, who said 816 were injured. The numbers were announced by the state run Xinhua news agency in an unusually swift revelation of the extent of the violence.

Police said the number of dead was expected to rise. State television said at least one member of the paramilitary People's Armed Police had been killed.

Uighur exile groups said the violence started when Chinese security forces cracked down on the peaceful protest.

It was only after dark and following several hours of violence that the paramilitary police, equipped with tear gas and firing weapons, were able to restore order.

The violence flared days after reports of ethnic clashes between Han Chinese and Uighur workers at a toy factory in the southern Guangdong province in which two Uighurs were killed and 188 wounded.

It is uncertain what sparked the riots, but they may have broken out around the time of the popular Sunday bazaar when thousands of Uighurs converge in towns across the region to sell their sheep, goats and horses.

Police have arrested several hundred participants, including more than "10 key figures who fanned the unrest," Xinhua said. The security bureau said police were still searching for 90 key figures suspected of being behind the single worst day of violence since troops crushed student demonstrations centred on Tiananmen Square in June 1989.

It gave no details as to whether those involved were members of the Uighur minority or whether the violence had been triggered by long-standing ethnic tensions in Xinjiang.

"We are extremely saddened by the heavy-handed use of force by the Chinese security forces against the peaceful demonstrators," said Alim Seytoff, vice-president of the Washington-based Uighur American Association.

"We ask the international community to condemn China's killing of innocent Uighurs. This is a very dark day in the history of the Uighur people," he said.

Xinjiang has been shaken by several riots against Chinese rule over the last several decades, although the violence had appeared to abate since the late 1990s.

Control has been particularly tight in Urunqi where Han Chinese are now believed to out-number the Uighurs.

Last year, just days before the Olympic Games opened in Beijing, two young Uighurs ploughed a truck into a group of border police who were on a morning run near their barracks in the fabled Silk Road city of Kashgar, killing 17.

Those men were arrested and later executed.

State media said the latest riot was not a spontaneous outburst but was incited by a small group of people intent on stirring up trouble. It gave no other details.