Reports of deaths varied and could not be independently confirmed. China's official Xinhua News Agency said 10 people -- including two hotel employees and two shop owners -- were burned to death, but that no foreigners were hurt. The exiled Tibetan government in India said about 100 were believed dead, citing unconfirmed sources.
Buddhist monks led the protests, which began Monday on the anniversary of a failed 1959 uprising against Chinese rule. The violent turn comes two weeks before China's Olympic celebrations kick into high gear with the start of the torch relay, which passes through Tibet.
Already the U.S. and other governments have urged China to show restraint toward the protesters, while International Olympic Committee President Jacques Rogge deferred, saying he didn't have details.
But a top official promised tough measures against detractors. "We will deal harshly with these criminals in accordance with the law," said Champa Phuntsok, chairman of the Tibetan government.
"Beating, smashing, looting and burning -- we absolutely condemn this sort of behavior. This plot is doomed to failure," said Phuntsok, an ethnic Tibetan, speaking on the sidelines of the National People's Congress, China's annual legislative session.
Law enforcement authorities in Tibet were offering leniency for demonstrators who surrender before Tuesday. Otherwise, they will be "severely punished," according to a notice carried on official Web sites and confirmed by prosecutors.
Phuntsok blamed the uprising on followers of the Dalai Lama, who fled into exile in 1959 after the failed uprising and is still Tibet's widely revered spiritual leader.
From Dharamsala, India, the Dalai Lama appealed to China not to use force, saying he was "deeply concerned," and urged Tibetans "not to resort to violence."
It is extremely difficult to get independent verification of events in Tibet since China maintains rigid control over the area. Foreigners need special travel permits, and journalists are rarely granted access except under highly controlled circumstances.
But footage and photos sent from Lhasa showed plumes of smoke billowing from buildings and small shops scattered across several parts of the city. Fire trucks moved through mainly empty streets after dark.
Earlier, eyewitness accounts and photos posted on the Internet portrayed chaos in Lhasa on Friday, with crowds hurling rocks at security forces, hotels and restaurants. The U.S. Embassy said Americans had reported gunfire.
The violence came on the fifth day of what had been mostly peaceful protests against China's often harsh 57-year rule over Tibet. After police tried to stop monks from protesting in central Lhasa, ordinary Tibetans vented pent-up anger on Chinese, torching shops and cars.
"The protesters yesterday went from attacking Chinese police to attacking innocent people very, very quickly," said a blog entry by a group of Westerners staying in a hotel in central Lhasa near the riot. "Many Tibetans were also caught in the crossfire."
Video footage on the web site showed Tibetan looking men attacking a motorcyclist, hitting him with rocks and pushing him over.
On Saturday, Xinhua said Lhasa had "reverted to calm."
"There was not much traffic on the road," the Xinhua report said. "Burned cars, motorcycles and bicycles remained scattered on the main streets, and the air is tinged with smoke."
Some shops were closed but government staff were required to work, said a woman who answered the telephone at the Lhasa Hotel. "There's no conflict today. The streets look pretty quiet," said the woman who refused to give her name for fear of retribution.
Tourists reached by phone or who arrived Saturday in Nepal described soldiers standing in lines sealing off streets where the rioting occurred. Armored vehicles and trucks ferrying soldiers drove the streets.
"Last night I saw 100 trucks of soldiers driving into the city. This morning I saw another 40 trucks of soldiers," said Plooij Frans, a Dutch tourist who left Lhasa Saturday by plane and arrived in Nepal's capital, Katmandu. "Every corner there were tanks, it would have been impossible to hold any protest today."
Government workers said they have been prevented from leaving their buildings. "We've been here since yesterday. No one has been allowed to leave or come in," said a woman who works for Lhasa's Work Safety Bureau, located near the Potala Palace, the former residence of the Dalai Lama.
The protests reverberated beyond China.
In Australia, pro-Tibet protesters clashed with police Saturday outside the Chinese consulate in Sydney, police said. Four demonstrators were arrested, said a police spokesman, who declined to give his name in line with departmental policy. Around 70 people took part in the protest.
In India, dozens of protesters launched a new march to Tibet on Saturday just days after more than 100 Tibetan exiles were arrested by authorities during a similar rally.
At a demonstration outside the United Nations in New York on Friday, Psurbu Tsering of the Tibetan Association of New York and New Jersey said its members received phone calls from Tibet claiming 70 people had been killed and 1,000 arrested. The reports could not be verified.
Over the centuries, Tibet was at times part of China's dynastic empires. Communist forces invaded the region in 1950, to reclaim the Himalayan region and seize the commanding heights overlooking rival India.
While activist groups and Western governments have said that China has not lived up to its promises of improving its human rights record for the Summer Games, the IOC has steadfastly refused to take a stand, saying the organization is not a political tool.
"It is not our job," Rogge, the IOC president, told reporters while visiting Puerto Rico. "We are not an activist organization."
Instead he said that he would issue a "reassuring message" Monday on Beijing's polluted air, which has been a concern for athletes.