Fires have broken out in the Tibetan city of Lhasa amid reports of rioting, as rare street protests led by Buddhist monks appeared to gather pace.
Monks began to protest on the streets of Lhasa earlier this week
One eyewitness told the BBC how large groups of people were setting fire to cars and shops and destroying anything of Chinese influence.
The US embassy in Beijing said US citizens had reported hearing gunfire.
Rallies have continued all week in what are thought to be the largest protests against Beijing's rule in 20 years.
Tibet's spiritual leader, the Dalai Lama, released a statement expressing deep concern, saying the protests were a "manifestation of the deep-rooted resentment of the Tibetan people".
The Dalai Lama, who heads Tibet's government-in-exile in India, called on the Chinese leadership to stop using force and begin dialogue with the Tibetan people.
He also urged Tibetans not to resort to violence.
"As I have always said, unity and stability under brute force is at best a temporary solution. It is unrealistic to expect unity and stability under such a rule," the statement said.
The US-based International Campaign for Tibet (ICT) said at least one police car had been set on fire on Friday.
ICT spokeswoman Kate Saunders said her group had received reports that the Tromsikhang market in Barkhor Street - a busy commercial neighbourhood - was either on fire or had burnt down.
China says Tibet always part of its territory
But Tibet enjoyed long periods of autonomy before 20th century
In 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
"It seems that lay people have now become involved in the protests," she said.
An eyewitness told the BBC there was a thick pall of smoke hanging over the city.
Another eyewitness said security forces and monks had clashed on Wednesday and several monks were beaten.
He said about 300 monks had tried to leave the Sera monastery to protest but security forces brandishing clubs stopped them and at least one monk was beaten to the ground.
The protests began earlier this week, when a number of monks were reportedly arrested after a march marking the 49th anniversary of a Tibetan uprising against Chinese rule.
Hundreds of monks took to the streets to demand their release - and reports say tear gas was used to disperse them.
The police were reported to have sealed off the city's three main monasteries on Thursday.
Correspondents say there is growing evidence that protests against Chinese rule are intensifying, despite assurances by Beijing on Thursday that the situation was under control.
China says Tibet has always been part of its territory, though Tibet enjoyed long periods of autonomy before the twentieth century, and many Tibetans remain loyal to the Dalai Lama, who fled in 1959 and currently lives in exile in India.
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