Clashes between protesters and security forces in Tibet's main city, Lhasa, have left at least two people dead, according to reports.
Stones were thrown and buildings burnt in Lhasa
An emergency official said that many people had been hurt and an unspecified number had died.
The US-based Radio Free Asia quoted witnesses who said they had seen at least two bodies on Lhasa's streets.
Tibet's government would "deal harshly" with the protesters, its Chairman Qiangba Puncog warned.
"We will deal harshly with these criminals who are carrying out activities to split the nation," he told the Associated Press news agency, denying that police had opened fire.
China's state-run Xinhua news agency earlier said police had fired warning shots and used tear gas to disperse protesters.
Rallies have continued all week in what are said to be the largest protests against Beijing's rule in 20 years.
A Western tourist in the city told the BBC: "[The rioters] seemed to go for all the Chinese shops and the Chinese people as well. I saw quite a few Chinese people beaten up... it turned totally crazy."
Another eyewitness said there were tanks on the street and he had seen people being carried away on stretchers.
British journalist James Miles, in Lhasa, told the BBC rioters took control of the city centre on Friday.
He some were looting shops and "taking out the contents and throwing them on huge fires which they've lit in the street".
China's government is braced for any further unrest on Saturday, with reports that a curfew is in place.
China says Tibet always part of its territory
Tibet enjoyed long periods of autonomy before 20th century
1950: China launched a military assault
Opposition to Chinese rule led to bloody uprising in 1959
Tibet's spiritual leader the Dalai Lama fled to India
Beijing accused the Dalai Lama's followers of "masterminding" the unrest, an allegation the Tibetan spiritual leader's spokesman labelled as "absolutely baseless".
From exile in India, the Dalai Lama expressed deep concern and called for an end to the violence.
He called on China to "address the long-simmering resentment of the Tibetan people through dialogue".
The rallies began earlier this week when a number of Buddhist monks were reportedly arrested after a march marking the 49th anniversary of a Tibetan uprising against Chinese rule.
In other developments on Friday:
- Hundreds of monks led a rally at Xiahe, in China's north-western Gansu province, the site of one of Tibetan Buddhism's most important monasteries
- More then 100 Tibetans in New York staged a protest outside the UN headquarters
- Police in New Delhi, India, clashed with protesters trying to reach the Chinese Embassy
- In Kathmandu, Nepal, police reportedly scuffled with some 1,000 demonstrators at a rally
BBC China editor Shirong Chen in Beijing says the Chinese government certainly does not want bloodshed - echoing that last September in Burma - five months before staging the Olympic Games.
On the other hand, they cannot allow the monks and other Tibetans to vent their anger in case this is seen as a sign of weakness, he says.
The US urged China to "respect Tibetan culture" and the American ambassador to China urged officials in Beijing to show restraint.
10 March: Monks arrested while marking 49th anniversary of failed uprising
11 March: Police use tear gas to disperse monks staging peaceful rallies
13 March: Police seal off key Lhasa monasteries
14 March: Rioting in Lhasa
A White House spokesman said: "The president has said consistently Beijing needs to have a dialogue with the Dalai Lama."
The European Union issued a statement urging China to address the concerns of Tibetans.
"We would like to see some kind of reconciliation between the Chinese authorities and the Tibetan representatives," said Dimitrij Rupel, foreign minister of Slovenia, which currently holds the EU's rotating presidency.
Hollywood actor Richard Gere - a supporter of Tibet - told the BBC he would back a boycott of the Beijing Olympics unless China adopted a new approach.
"The Tibetans have been marginalised in recent years, much more than before, and that's why you see this pressure cooker effect," he said.
China says Tibet has always been part of its territory - although Tibet enjoyed long periods of autonomy before the 20th Century and many Tibetans remain loyal to the Dalai Lama, who fled in 1959.
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