"Hundreds of people, including monks and civilians, are in the protest."
Tensions in Lhasa have increased in recent days as the city's three biggest monasteries were sealed off by thousands of soldiers and armed police amid the largest protests in nearly two decades.
Rights groups say this week's demonstrations were the biggest protests in Tibet since Chinese authorities declared martial law to quell a wave of pro-independence demonstrations by monks in 1989.
According to Radio Fee Asia (RFA), a US-funded radio service, the violence came as Buddhist monks started a hunger strike and two others attempted suicide.
Human rights and Tibetan exile groups fear a violent crackdown after the rare anti-Chinese protests.
Tashi Choephel, a researcher at the Tibetan Centre for Human Rights and Democracy in Dharamsala, India, told Al Jazeera that more than a hundred monks held a peaceful demonstration on Friday.
"But they were then surrounded and cordoned off by the People's Armed Police [Chinese police] and security officials," he said.
"Afterwards, a scuffle ensued, and led to the burning of cars and shops."
Choephel said that demonstrations are happening in other Tibetan towns as well.
Soldiers moved into position around the sites on the third day of protests involving hundreds of Buddhist monks.
Chinese authorities reportedly used tear gas and electric prods to disperse hundreds of protesters, and detained up to 50 monks.
Tourists are reportedly being barred from entering the monasteries.
Earlier this week, 500 monks from the Drepung monastery staged a march in the capital, followed by protests of monks at Lhasa's Sera and Ganden monasteries.
On Thursday, Chinese government officials confirmed reports of protests in Lhasa but said the situation had been "stabilised".
According to RFA, monks from Sera are demanding the withdrawal of Chinese paramilitary forces from the monastery compound and the release of monks detained earlier this week.
RFA also reported that two monks from another monastery were in critical condition after attempting suicide by slitting their wrists.
But a spokesman for the Tibet Autonomous Region - as it is officially known in China - denied all the claims.
He said there were no arrests or troops surrounding the monasteries, adding that protests had not spread to the rural areas and that the monasteries were open to tourists.
The Dalai Lama, Tibet's spiritual leader, fled into exile in India after an uprising in 1959, nine years after the invasion of Chinese troops.
This week marks the 49th anniversary of the failed uprising, with protests being held in major Asian capitals.
Many Tibetans - including those who say they do not want full independence - are angry at what they see as the suppression of their culture.
The latest show of Tibetan defiance is likely to worry China's leadership as it seeks to secure a stable environment in the run-up to the Olympic Games in Beijing in August.
Source: Al Jazeera and agencies