Violence in China's restive western region of Xinjiang has left at least 140 people dead and more than 800 people injured, state media say.

Several hundred people were also arrested after a protest turned violent in the city of Urumqi on Sunday.

Beijing says Uighurs went on the rampage but one exiled Uighur leader says police fired on students.

The protest was reportedly prompted by a deadly fight between Uighurs and Han Chinese in southern China last month.

The BBC's Chris Hogg in Shanghai says that if the numbers of dead are to be believed - and state media say they may rise - this look like the bloodiest violence in China since Tiananmen Square 20 years ago.

It is still unclear who died in the violence and why so many were killed.

'Foreign plot'

Uighur exiles said police had fired indiscriminately on a peaceful protest in Urumqi.

The Xinjiang government blamed separatist Uighurs based abroad for orchestrating attacks on ethnic Han Chinese.

Eyewitnesses said the violence started on Sunday with a few hundred people, and grew to more than 1,000.

Xinhua says the protesters carried knives, bricks and batons, smashed cars and stores, and fought with security forces.

Wu Nong, news director for the Xinjiang government, said more than 260 vehicles were attacked and more than 200 shops and houses damaged. An overnight curfew was imposed.

BBC sources in China report they have been unable to open the Twitter messaging site in Shanghai and that message boards on Xinjiang on a number of websites were not taking posts.

Reports from Xinjiang suggest some internet and mobile phone services have been blocked.

Urumqi resident Han Zhenyu told Reuters news agency there was no access to the internet. A representative of the China Mobile phone service told Associated Press its service was suspended in the region.

'Dark day'

Uighur groups insisted their peaceful protest had become victim to state violence.

UIGHURS AND XINJIANG Uighurs are ethnically Turkic Muslims They make up about 8m of the 20m population China re-established control in 1949 after crushing short-lived state of East Turkestan Since then, large-scale immigration of Han Chinese Uighurs fear erosion of traditional culture Sporadic violence since 1991 In pictures: Xinjiang protests Q&A: China and the Uighurs

The Uighurs were reportedly angry over an ethnic clash last month in the city of Shaoguan in southern Guangdong province.

A man there was said to have posted a message on a local website claiming six boys from Xinjiang had "raped two innocent girls".

Police said the false claim sparked a vicious brawl between Han and Uighur ethnic groups at a factory. Two Uighurs were killed and 118 people were injured.

However, the Xinjiang government has blamed the latest unrest on businesswoman Rebiya Kadeer, the Uighurs' leader who is living in exile in the United States.

"An initial investigation showed the violence was masterminded by the separatist World Uighur Congress led by Rebiya Kadeer," the government said in a statement, according to Xinhua.

It said the violence had been "instigated and directed from abroad".

FROM THE TODAY PROGRAMME   More from Today programme

The vice-president of the US-based Uighur American Association, Alim Seytoff, condemned the "heavy-handed" actions of the security forces.

"We ask the international community to condemn China's killing of innocent Uighurs. This is a very dark day in the history of the Uighur people," he said.

The BBC's Quentin Sommerville in China says Xinjiang, a mainly Muslim area, has been a source of tension for many years.

Some of its Uighur population of about eight million, want to break away from China, and its majority Han Chinese population.

The authorities say police are securing order across the region and anyone creating a disturbance will be detained and punished.