Nearly 1,000 protesters from a Muslim ethnic group rioted in China’s far west, overturning barricades, attacking bystanders and clashing with police in violence that killed at least four people, including a policeman, state media and witnesses said.

Protesters, mostly from the Uighur ethnic group, set dozens of cars on fire and attacked buses in several hours of violence in the Xinjiang province city of Urumqi. The violence appeared to subside as the police and military presence intensified into the night, according to participants and witnesses.

Tensions between Uighurs and the majority Han Chinese are never far from the surface in Xinjiang, China’s vast Central Asian buffer province, where militant Uighurs have waged sporadic, violent separatist campaign. The overwhelming majority of Urumqi’s 2.3 million people are Han Chinese.