The rioting broke out Sunday afternoon in a large market area of Urumqi, the capital of Xinjiang province, reports the New York Times.
It lasted for several hours before riot police officers and paramilitary or military troops locked down the Uighur quarter of the city.
Among the dead were three Han Chinese and one police officer.
Uighur men were led into nearby police stations with their hands behind their backs and shirts pulled over their heads, one witness said.
Early Monday, the local government announced a curfew banning all traffic in the city until 8 p.m.
The riot was the largest ethnic clash in China since the Tibetan uprising of March 2008, and perhaps the biggest protest in Xinjiang in years.
According to a Xinhua news report, initial investigation showed the World Uyghur Congress led by Rebiya Kadeer masterminded the unrest.
According to the government, the World Uyghur Congress has recently been instigating unrest via the Internet among other means, calling on the outlaws "to be braver" and "to do something big."
Nur Bekri, chairman of the Xinjiang regional government, said in a televised speech Monday morning that the movement came after a conflict between Uygur and Han ethnic people in a toy factory in the southern Guangdong province on June 26.
Two Uygur workers were killed during the factory brawl, which was triggered by a sex assault by a Uygur worker toward a Han female worker. A total of 120 others of both Han and Uygur ethnic groups were injured.
Nur Bekri said some overseas opposition forces instigated Sunday's unrest to undermine ethnic unity and social stability in the autonomous region.
"We should bear in mind that stability is to the greatest interest of all people in China, including the people in the Xinjiang Uygur Autonomous Region," he said.(ANI)