Rights groups and exiles say police often use often heavy-handed tactics against the Muslim Uighur community, which calls Xinjiang home. Violence has broken out previously when groups of Uighurs protest at police stations, they say.
China has stepped up security in Xinjiang after a vehicle ploughed into tourists on the edge of Beijing's Tiananmen Square in October, killing three people in the car and two bystanders.
China said the attack was carried out by Islamist militants. Many of Xinjiang's Turkic-speaking, Muslim people chafe at restrictions on their culture, language and religion, though the government insists it grants them broad freedoms.
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Xinjiang has been the scene of numerous incidents of unrest in recent years, which the government often blames on the separatist East Turkestan Islamic Movement, even though many experts and rights groups cast doubt on its existence as a cohesive group.
Many rights groups say China has long overplayed the threat posed to justify its tough controls in energy-rich Xinjiang, which lies strategically on the borders of Central Asia, India and Pakistan.