At least 140 people were killed and more than 800 injured Sunday, according to China's official news agency, in an explosion of ethnic violence in Urumqi, capital of China's restive Xinjiang Uyghur Autonomous Region. The reports gave no details of how the deaths were caused.

Riots and street-battles between Muslim Uyghurs and Han Chinese capped weeks of rising tensions between the two groups, and renewed concerns about China's treatment of its ethnic minorities. The escalating death toll in the far-west region makes this the most violent mass incident in China in many years.

"This violence is a clear demonstration that policies that are meant to give human rights to all the ethnic minorities in the region are not working," Roseann Rife, Amnesty International's deputy program director for Asia, told Asia Times Online on Monday.

The rioting began with a peaceful protest against the government's handling of a clash last month between Han Chinese and Uyghur workers at a toy factory in Shaoguan in southern Guangdong province in which two Uyghurs were killed.

Uyghurs are the majority ethnic group in Xinjiang, making up about half the 20 million population. Many Uyghurs resent what they see as China's repression of their culture and policies that they say favor Han Chinese, the country's dominant ethnic group.

The Beijing government has accused Uyghur exiles of orchestrating the violence in Urumqi, but Uyghur spokesmen claim they were holding a peaceful sit-in that became violent only after government troops initiated force. State television described ensuing events as a rampage of "beating, smashing, robbing and burning".

By late Monday, a local police official in Urumqi said the death toll was still climbing and armed police were patrolling city streets. Firefighters were struggling to put out flames raging through the city.

At around 7pm on Sunday, hundreds of Uyghurs took to the streets with knives, wooden batons, bricks and stones, Xinjiang police chief Liu Yaohua told a press conference. Rioters also vandalized vehicles and buildings and fought with ranks of anti-riot police, Liu said.

According to a preliminary investigation, 203 shops and 14 homes were destroyed in the riots along with 216 motor vehicles, including 190 buses, 10 taxis and two police cars. Residents in Urumqi, a city of 2.3 million, have told Chinese media they still feel at risk even as order was being slowly restored. Rights officials believe they may have good reason to fear.

"This is likely to result in a harsh crackdown. You're going to see an escalation of security measures in the region and a clampdown on everything - meetings, religious gatherings, writings, the media - it will be across the board," said Amnesty International's Rife.

Urumqi residents were unable to access the Internet on Monday, several said. "The city is basically under martial law," Yang Jin, a dried fruit merchant, told news agency Reuters. The government appeared to have curbed Internet access as far afield as Beijing and Shanghai, Reuters reported, saying social networking site Twitter had also been blocked., a domestic competitor of Twitter, was still accessible, though searches for keywords such as "Urumqi," "Xinjiang" and "Uyghur" gave no results, the report said.

Xinjiang governor Nuer Baikeli, a Uyghur, said in a speech shown on Xinjiang television that the unrest was the work of extremist forces abroad.

"After the Shaoguan incident, the three forces abroad strove to whip this up and seized it as an opportunity to attack us, inciting street protests," Baikeli said. The "three forces" refer to groups the government says engage in separatism, militant action and religious extremism.

An unnamed Chinese official was quoted by Xinhua as saying that the "unrest was masterminded by the World Uyghur Congress led by Rebiya Kadeer".

"This was a crime of violence that was pre-meditated and organized," said the Xinhua report.

Kadeer is a Uyghur businesswoman now in exile in the United States after years spent in China's penal system on charges of involvement in separatist activities. Uyghur groups have rejected the Chinese government claim of a plot, claiming the riot this weekend was an outpouring of pent-up anger over government policies and Han Chinese dominance of economic opportunities.

"They're blaming us as a way to distract the Uyghurs' attention from the discrimination and oppression that sparked this protest," Dilxat Raxit, a spokesman for the World Uyghur Congress in exile in Sweden told Reuters.

Raxit added that tens of thousands of demonstrators had gathered in every Uyghur neighborhood in Urumqi to protest peacefully against what he described as the government's ethnic cleansing in Shaoguan.

After about 40 minutes during which the crowd shouted slogans, the Chinese military began its crackdown by sending more than 50 military vehicles - including tanks - carrying troops into Urumqi.
"According to Chinese law, people have the right to protest peacefully," the World Uyghur Congress said in an appeal. "We call for attention to this kind of ethnic discrimination."

Police have arrested several hundred in connection with the riot, including more than 10 key players who fanned unrest, Liu said. He said police are still searching for about 90 other key suspects in the city.

"Police have tightened security in downtown Urumqi streets and at key institutions such as power and natural gas companies and TV stations to prevent large-scale riots," Liu said, adding that checkpoints have been set up in Urumqi's key areas as well the neighboring Changji and Turpan prefectures to prevent the rioters from fleeing, Liu said.

He said more than 100 ethnic officials from adjacent areas have been transferred to Urumqi to interrogate the suspects.

The Uyghur diaspora community portrays their people as a community pit against an oppressive and unaccountable Chinese government. Many Uyghurs resent what they see as China's repression of Uyghur culture and policies they say favor Han Chinese, the country's dominant ethnic group.

While there is no uniform Uyghur agenda, the desired outcome by groups that use violence is broadly a separate Uyghur state that lays claim to a large part of western China and some territory in neighboring Central Asian republics. The root causes of dissatisfaction are a mix of history, ethnicity, and religion, fueled by poverty, unemployment, social disparities, and political grievances.

Resource-rich but sparsely populated Xinjiang, which covers an area more than three times the size of California, borders Russia, Mongolia, Kazakhstan, Kyrgyzstan, Tajikistan, Afghanistan, Pakistan and India.