FACTBOX: China's restive Xinjiang region

(Reuters) - Hundreds of Uighur Muslims crowded into at least one mosque in riot-stricken Urumqi on Friday after authorities relented on a decision to close mosques for the main day of prayer to minimize ethnic tensions.

Security forces have imposed control over Urumqi, but the prayers after midday will be a test of the government's ability to contain Uighur anger after Han Chinese, China's predominant ethnic group, attacked Uighur neighborhoods on Tuesday.

Those attacks were in revenge for the deaths of 156 people in Uighur rioting on Sunday, the region's worst ethnic violence in decades.

Here are some facts about the region.

* Xinjiang, China's largest provincial-level administrative unit by area, covers one sixth of the country. It is relatively sparsely populated with around 20 million people.

* It is home to 8 million Uighurs, a Turkic, largely Islamic people who share linguistic and cultural bonds with Central Asia. Many resent the Han Chinese economic dominance in Xinjiang.

* The northern part of Xinjiang is economically dominated by the "bingtuan," military-run farms and businesses that predominantly employ Han Chinese settlers. In southern Xinjiang, where Uighurs are still the majority, China recently announced plans to raze the historic quarters of Kashgar, an oasis city.

* The Uighur language has been largely phased out of higher education and Uighurs are limited in their ability to travel independently to Mecca for the annual Haj. By contrast, China's central government has supported Islamic studies and Haj travel for the Hui, a Muslim people culturally akin to the Han.

* Along with Tibet, Xinjiang is one of the most politically sensitive regions in China. In both cases China says its rule has brought economic growth and prosperity.

* Xinjiang is strategically located at the borders of Russia, Mongolia, Kazakhstan, Kyrgyzstan, Tajikistan, Afghanistan, Pakistan and India. It has abundant oil reserves and is China's largest natural gas producing region.

* The oasis cities in what is now Xinjiang were conquered by China during the Han dynasty. For the next two millennia, they were variously independent, under Chinese rule, or part of other central Asian kingdoms. The area was briefly an independent East Turkestan in the 1940s and has been ruled by Beijing since the Communist victory in 1949.

* Chinese security officials blamed attacks before and during last year's Olympic Games on independence-seeking Uighur militants. In the most violent, 16 armed police were killed in a bomb and stabbing attack in Kashgar.

* The Chinese government has accused militant Uighurs of working with Islamist militant group al Qaeda to bring about an independent East Turkestan by violent means.

* Human rights groups say China has used its support for the U.S.-led fight against al Qaeda to justify a wider crackdown on Uighurs, including arbitrary arrests, closed-door trials and application of the death penalty.

* Seventeen Uighur men were detained at the U.S. military prison at Guantanamo Bay, after being captured by U.S. troops in Afghanistan. Reluctant to return them to China, the United States has had trouble convincing other countries to take them.

(Reporting by Lucy Hornby and Ben Blanchard; Editing by Nick Macfie)