Violent protests erupted Friday in a busy market area of Lhasa, the Tibetan capital, as Buddhist monks and other ethnic Tibetans clashed with Chinese security forces. Witnesses say the protesters burned shops, cars, military vehicles and at least one tourist bus.

The chaotic scene marked the most violent demonstrations since protests by Buddhist monks began in Lhasa on Monday, the anniversary of a failed Tibetan uprising against Chinese rule in 1959. The protests have been the largest in Tibet since the late 1980s, when Chinese security forces repeatedly used lethal force to restore order in the region.

The developments prompted the Dalai Lama, the exiled spiritual leader of Tibetan Buddhism, to issue a statement, saying he was concerned about the situation and appealing to the Chinese leadership to “stop using force and address the long-simmering resentment of the Tibetan people”.

By Friday night, Chinese authorities had placed much of the central part of the city under a curfew, including neighborhoods around different Buddhist monasteries, according to two Lhasa residents reached by telephone. Military police were blocking roads in some ethnic Tibetan neighborhoods, several Lhasa residents said.

Meanwhile, the United States Embassy in Beijing warned American citizens to stay away from Lhasa. The embassy said it had “received firsthand reports from American citizens in the city who report gunfire and other indications of violence.”

The Chinese government’s official news agency, Xinhua, issued a two-sentence bulletin, in English, confirming that shops in Lhasa had been set on fire and that other stores had closed because of violence on the streets. But the Chinese news media otherwise carried no news about the protests. The White House responded with expressions of concern, but not direct criticism, although it urged the Chinese authorities to use restraint.

“We believe Beijing needs to respect Tibetan culture, needs to respect multi-ethnicity in their society,” a spokesman, Tony Fratto, said while traveling with President Bush to New York. “We regret the tensions between ethnic groups and Beijing.”

The White House says that the American ambassador in Beijing, Clark T. Randt Jr., had urged restraint in his contacts with the Chinese authorities.

The disturbances appear to be becoming a major problem for the ruling Communist Party, which is holding its annual meeting of the National People’s Congress this week in Beijing. China is eager to present a harmonious image to the rest of the world as Beijing prepares to play host to the Olympic Games in August.