State media initially said at least three ethnic Han Chinese were killed in the violence yesterday, though later reported that an unknown number died, among them an armed policeman. An activist group said one demonstrator may have died.

Protesters, mostly from the Uighur ethnic group, set at least one car on fire, overturned police barriers and attacked buses in several hours of violence that appeared to subside somewhat as police and military presence intensified into the night, according to participants and witnesses.

Tensions between Uighurs and Chinese are never far from the surface in Xinjiang province, China's vast Central Asian buffer province, where militant Uighurs have waged a sporadic, violent separatist campaign.

Protesters gathered yesterday to demand an investigation into a fight between Uighur and Han Chinese workers at a factory in southern China last month. Accounts differed over what happened next in the city of Urumqi, but the violence seemed to have started when a crowd of protesters - who started out peaceful - refused to disperse.

Adam Grode, an American Fulbright scholar studying in Urumqi, said he heard explosions and also saw a few people being carried off on stretchers and a Han Chinese man with blood on his shirt entering a hospital.

He said he saw police pushing people back with tear gas, fire hoses and batons, and protesters knocking over police barriers and smashing bus windows.

"Every time the police showed some force, the people would jump the barriers and get back on the street. It was like a cat-and-mouse sort of game," said Grode, 26.

People started to disperse after two hours, he said, but hundreds of police and soldiers poured into the city in the night with two dozen police buses, trucks, and other security vehicles and rounded up Uighurs who were sitting on street curbs.

The government's Xinhua News Agency quoted unnamed officials saying that at least three ethnic Han Chinese were killed in the violence, in which the crowd attacked passers-by, torched vehicles and interrupted traffic on some roads. It later said an unknown number of people were killed, including the policeman.

Xinjiang's government accused Uighur exiles led by a former businesswoman now living in America, Rebiya Kadeer, of fomenting the violence via the Internet.

Dilxat Raxit, a spokesman for the pro-independence World Uighur Congress based in Germany, said he received calls from Urumqi describing the protest as peaceful until police used force to try to clear the square.

Dilxat said some protesters were beaten badly. One of his informants told him that one person was killed. The account could not immediately be corroborated.

Uighur separatists have waged a sporadic campaign for independence in recent decades.

Four Uighur detainees at the U.S. prison at Guantanamo Bay in Cuba were recently released and relocated to Bermuda despite Beijing's objections because U.S. officials have said they fear the men would be executed if they returned to China. Officials have also been trying to transfer 13 others to the Pacific nation of Palau. *