Chinese troops and police have been deployed at important monasteries in Tibet to quell the biggest protests by Tibetan Buddhist monks in the Himalayan region for nearly 20 years.

Witnesses have reported trucks full of troops surrounding Drepung monastery in Lhasa, while Sera monastery was ringed by hundreds of police.

These two sites have strong symbolic significance, as they were the training grounds for the monks who led Tibet before the People's Liberation Army came in 1950 and ousted the Dalai Lama.

Protests began on Monday as monks marked the 49th anniversary of the failed uprising against Chinese rule that culminated in the Dalai Lama's exile. The protests are the biggest since the late 1980s, when riots led to martial law. Back then, China's current President, Hu Jintao, was the Communist Party chief in Tibet.

Signs of defiance in Tibet come just five months before the Olympic Games in Beijing, when the eyes of the world will be on China. Tibetan activists are expected to use the extra attention to highlight their cause.

Chinese officials in Beijing confirmed protests had taken place, but said the situation had stabilised. They accused the Dalai Lama of provoking separatist activities.

"In recent days, a few monks in Lhasa city have made some disturbances. This is a political scheme by the Dalai group, attempting to separate China and try to make some unrest in the normal harmonious, peaceful life of Tibetan people," a Foreign Ministry spokesman, Qin Gang, told a briefing, adding that further protests would "not take place" because of China's determination to safeguard its national interest.

So far as Beijing is concerned, Tibet is part of its inviolable territory and always has been. Beijing stresses the role it has played in bringing economic well-being.

This week, protests have rippled across the mountainous enclave. More than 300 monks from Drepung marched on Monday, while a smaller group from Sera also protested. A number of arrests were made. Radio Free Asia reported that police used tear gas to disperse 500 to 600 monks from Sera monastery who were marching to demand the release of imprisoned fellow monks.

The Free Tibet Campaign also said its contacts had spoken to Tibetans who reported demonstrations by 400 monks in Lutsang monastery in Qinghai province, an area Tibetans call Amdo, as well as at the Myera monastery in Gansu.

"The reports of protests outside Lhasa show Tibetans know the eyes of the world are upon them and are determined not to let the momentum drop. Tibetans inside Tibet are aware that Tibetans in India are marching towards the Tibet border and have been emboldened by the support they are receiving from across the world," said Matt Whitticase of the Free Tibet Campaign.

More than 100 Tibetan exiles were dragged away by police in northern India yesterday, and promptly began a hunger strike after being charged with threatening the "peace and tranquillity" of the region.

Clutching Tibetan flags and pictures of Gandhi and the Dalai Lama, they had planned to march from India to Tibet to coincide with the start of the Olympics. Their first setback came at the beginning of the week, when Indian officials barred them from leaving the outskirts of Dharmasala, the headquarters of the Tibetan government-in-exile. Yesterday they had marched about 12 miles beyond the boundary when Indian police stopped them in their tracks.