He also told reporters on the sidelines of China's annual parliamentary session: "The plot of the separatists will fail. We will challenge them firmly, according to law."
Radio Free Asia, a US-funded broadcaster, had reported that two people were killed in the clashes.
An official with the Lhasa's emergency medical centre told the AFP news agency: "We are very busy with the injured people now - there are many people injured here.
"Definitely some people have died, but I don't know how many."
Many police officers were badly injured in the clashes, Chinese state media said, while also reporting that a mosque and other buildings had been set on fire.
The protests, which began on Friday morning, were led by 100 monks and quickly attracted other Tibetans.
Witnesses reported hearing gunfire and seeing vehicles in flames in the city's main Barkor shopping district.
Crowds hurled rocks at security forces and at restaurant and hotel windows.
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.
Tashi Choephel, a researcher at the Tibetan Centre for Human Rights and Democracy in Dharamsala, told Al Jazeera that more than one 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."
Xinhua, the state news agency, said a number of shops, banks and hotels were burnt down, causing blackouts and telecommunications interruptions in some areas.
It said that the regional government "took emergency measures to rescue residents under attack," including reinforcing the protection of schools, hospitals and gas stations.
Choephel said that demonstrations were also happening in other Tibetan towns.
Hundreds of monks of Labrang monastery in the northwestern province of Gansu led a march through the town of Xiahe, the Free Tibet Campaign said, citing sources in Dharamsala.
Also on Friday, about a dozen Tibetan exiles in India were arrested when they tried to storm the Chinese embassy in New Delhi.
The Dalai Lama, Tibet's spiritual leader, urged China not to use force against protesters.
"I ... appeal to the Chinese leadership to stop using force and address the long-simmering resentment of the Tibetan people through dialogue with the Tibetan people," he said in a statement issued from his base in Dharamsala.
|About a dozen Tibetan activists were arrested|
at the Chinese embassy in New Delhi [AFP]
"I also urge my fellow Tibetans not to resort to violence."
China accused him of masterminding the protests.
The Tibet regional government said there had been enough evidence to prove that the troubles in Lhasa were "organised, premeditated and masterminded by the Dalai clique".
A spokesman for the Dalai Lama said the allegations are "absolutely baseless".
The Dalai Lama 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.
On the diplomatic front, Clark Randt, the US ambassador to China, has told Beijing to act with restraint when dealing with protesters and urged China to engage in dialogue with the Dalai Lama.
Sean McCormack, a state department spokesman, said Randt had spoken to Chinese officials.
"He took the opportunity, because of what was going on in Lhasa, to urge restraint on the part of the Chinese officials and Chinese security forces and not resort to use of force in dealing with the protesters," McCormack said.
EU leaders have also called for Chinese authorities to show "restraint".
"We have called very clearly that human rights be assured ... the condemnation is high," Bernard Kouchner, the French foreign minister, said after a EU summit in Brussels.
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.
Tensions in Lhasa have increased after the city's three biggest Tibetan Buddhist monasteries were sealed off by thousands of soldiers and armed police.
Rights groups say the demonstrations are the region's biggest since Chinese authorities declared martial law to quell a wave of pro-independence demonstrations by monks in 1989.
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.
Source: Al Jazeera and agencies