Chinese authorities tightened security around Tibet's main monasteries and banned visits to a sacred site on the edge of the capital, Lhasa, for fear of a fresh outburst of unrest on the Dalai Lama's birthday.

Few monks remain, however, in the province's three most important monasteries. Many have disappeared, their whereabouts a mystery. Chinese officials have deployed troops and paramilitary police around the ancient religious institutions, suspecting these sprawling hillside communities are at the heart of the unrest that has swept the region since early March.

Dozens, possibly several hundred, have been arrested or are detained and under investigation for their roles in the anti-Chinese demonstrations and riots that hit Lhasa on March 14. This, however, does not account for the empty halls in the three great monasteries, Drepung, Sera and Ganden, that lie near the city. Several hundred monks are believed to have been living in each of them before the violence erupted.

Now Tibetan sources have revealed that most of the monks, more than 1,000 in total, have been transferred to many prisons and detention centres in and around the city of Golmud in neighbouring Qinghai province. The detained monks are all young ethnic Tibetans from surrounding regions who had made their way to Lhasa, their spiritual capital, to study and pray in the most prestigious spiritual centres on the Roof of the World.

Their detention is part of a policy to rid the monasteries of any monks not registered as formal residents of the administrative region, known as the Tibetan Autonomous Region.

Family members say that the monks have been told they will be incarcerated in Golmud only until the end of the Olympic Games in Beijing. The policy is part of a campaign by the Chinese Government to ensure that the Games, opening on August 8 and lasting for two weeks, pass off without a hitch and without protests from the restive Tibetans, they told The Times.

“After that they have been told that they will be allowed to leave, because they are not guilty of a crime,” one man whose brother is among the detained said. “But they will be ordered to return to their home villages and will not be permitted to go back to the monasteries in Lhasa.” There were no reports that any of those being held were being mistreated, he added.

The monks' detention is, in effect, a decision by China to implement a policy first promulgated in 1994 to limit the size of Tibet's monasteries, because increasing religious freedoms were attracting growing numbers.

Sera monastery, for example, is supposed to house no more than 400 monks but is believed to have grown to more than 1,000. In Drepung - at its height the largest monastery in the world - has been allocated a similar quota but has allowed as many as 900 monks to live in its high-walled compounds. The monasteries have for years allowed young boys well below the age of 18 to enter in direct contravention of the rules but the authorities had turned a blind eye.

The abbots have encouraged the unofficial monks because they found that those from other regions tended to be the most devoted and diligent, Tibetan sources said.

Registered monks are given a monthly stipend that can sometimes be as much as 5,000 yuan (£350) depending on the donations to a monastery and entrance ticket sales. Many prefer to spend their days playing video games and DVDs rather than reading the scriptures, they said. They voiced concern that the monasteries could lose many of their best Buddhist scholars if the monks were not allowed to return after the Olympics.

Authorities have ordered all Tibetans without a Lhasa residence permit to leave the city and to return to their homes. Reports are increasing of the authorities targeting individuals whose dress, haircut or even teeth - Tibetans from Sichuan and Qinghai favour gold fillings - mark them out as coming from neighbouring regions.

Tibetan residents of Lhasa say that they prefer not to wear Tibetan dress for fear they will be stopped and questioned on the street by police or soldiers. Men say that they are growing their hair so as not be mistaken for a monk and interrogated.

One man, from the Khamba group that lives in western Sichuan province and is renowned for its warriors, told how he was arrested after the March 14 riot because his long hair identified him as being from that region.

The huge security operation has,however, failed to halt protests by Tibetans demanding the return of the Dalai Lama and independence for their homeland. On June 18 six took place in Ganze county alone.

One Tibetan source said: “They know they are going to be arrested but people still go out and demonstrate. And then you can see the cats come out and catch them like mice.”

Ancient traditions

Drepung monastery

The largest of Tibetan monasteries, whose name means “rice heap”. Its population numbered as many as 7,700 in the 1930s and sometimes up to 10,000. Founded in 1416, it has long been been regarded as the most academic monastery of the Gelukpa – or Yellow Hat – sect

Sera monastery

Its name means “Enclosure of Roses”. Also founded in the early 15th century. Began as a scholarly institution but became known as the home of warrior monks whose responsibility was to defend Tibet and its Buddhist traditions

Ganden monastery

The oldest of the three great monasteries, its name means “continent of completely victorious happiness”. It is 35 miles from Lhasa and has long been the smallest house. It suffered most during the 1966-76 Cultural Revolution, when it was dynamited by the Army and Red Guards