Violence in China's restive western region of Xinjiang has left at least 156 people dead and more than 800 people injured, state media say.
Several hundred people were arrested after a protest, in the city of Urumqi on Sunday, turned violent.
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 says the violence is some of the worst reported in the country since Tiananmen Square in 1989.
'Dark day'
Eyewitnesses said the violence started on Sunday in Urumqi after a protest of a few hundred people 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.
Most of the violence is reported to have taken place in the city centre, around Renmin (People's) Square, Jiefang and Xinhua South Roads and the Bazaar.
See detailed map of Urumqi city centre
The police presence was reported to be heavy on Monday.
Adam Grode, an American studying in Urumqi, told Associated Press: "There are soldiers everywhere, police are at all the corners. Traffic has completely stopped."
A witness in the Xinjiang city of Kashgar told AP there was a protest there on Monday of about 300 people but there were no clashes with police.
It is still unclear who died in Urumqi and why so many were killed.
The Xinjiang government blamed separatist Uighurs based abroad for orchestrating attacks on ethnic Han Chinese.
But Uighur groups insisted their protest was peaceful and had fallen victim to state violence, with police firing indiscriminately on protesters in Urumqi.
Dolkun Isa, a spokesman for the World Uyghur Congress (WUC) in Munich, disputed the official figures, saying the protest was 10,000 strong and that 600 people were killed.
He rejected reports on Xinhua that it had instigated the protests.
Xinhua had quoted the Xinjiang government as blaming WUC leader Rebiya Kadeer for "masterminding" the violence.
But Mr Isa said the WUC had called on Friday only for protests at Chinese embassies around the world.
More than 260 vehicles were destroyed in Urumqi, officials said
Alim Seytoff, the vice-president of another Uighur group - the US-based Uighur American Association - 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.
When asked about the rioting, UN Secretary General Ban Ki-moon said that all governments must protect freedom of speech and "the life and safety of civilian populations".
A spokesman for UK PM Gordon Brown said Britain was urging "restraint on all sides".
Italian President Giorgio Napolitano said he had raised the issue of human rights with visiting Chinese President Hu Jintao in Rome.
Internet blocks
The Uighurs in Urumqi 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.
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.
Analysts say the government's so-called Great Firewall of China, which it uses to block unwanted internet material, will prevent large-scale dissemination of information but that dedicated internet users can bypass it fairly easily.
BBC China editor Shirong Chen says there has been ethnic tension in Xinjiang since before the founding of the People's Republic.
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.
However, our China editor says there may be questions asked about their inability to prevent a protest they knew about days in advance
BBC NEWS | Asia-Pacific | Scores killed in China protests
..post it in a systematic order okz 
