They were the 15 youthful Tibetan monks – three still in their teens – who sparked a rebellion by daring to speak out against China's repression of their homeland.

The group paraded peacefully down Barkhor Street in Lhasa old town on 10 March handing out leaflets, chanting pro-independence slogans and carrying the banned Tibetan flag. Their demand was that the Chinese government that has ruled Tibet since 1951 should ease a "patriotic re-education" campaign which forced them to denounce the Dalai Lama and subjected them to government propaganda.

The reaction of the authorities, desperate to snuff out the most serious uprising against Chinese rule for almost half a century, was rapid and brutal. The group was detained on the spot, with eyewitnesses reporting that several of the monks suffered severe beatings as they were arrested and taken away. They have not been seen since.

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Amnesty International called last night for their immediate release, along with all the other anti-Chinese demonstrators picked up in the past three weeks. The human rights organisation said they were at "high risk of torture and other ill treatment" and called on supporters to write to Hu Jintao, the Chinese President, with copies to the Chinese embassy in London.

Steve Ballinger, a UK spokesman for Amnesty, said: "China's reaction to peaceful protests in Tibet and neighbouring provinces – detaining demonstrators, flooding the area with troops and reportedly using violence – does not bode well for the Olympics. Some protests may have turned violent and the Chinese authorities have a responsibility to protect the lives and property of people in the region. But locking up peaceful protesters and locking out journalists is totally unacceptable. These monks must be released immediately and all those detained in recent weeks must be accounted for. If basic human rights are not respected, China's promises to clean up its act ahead of the Olympics will seem very hollow indeed."

The Tibetan Centre for Human Rights and Democracy (TCHRD), which operates in exile from India, expressed its "deepest fear" that monks face "extreme inhumane treatment" in Chinese detention centres. It said: "Torture is a regular exercise in Chinese-administered prisons and detention centres in Tibet."

The plight of the monks was being seen as a key symbolic test for the Chinese government as it tries to bring calm to the country before this summer's Olympic Games in Beijing. Yesterday the Olympic flame was lit in Greece and began a global journey to the Olympic stadium in Beijing. But its progress risks being overshadowed by protests if China continues apparently to ignore the human rights of those who protest against it.

The monk's march – on the 49th anniversary of a failed uprising against Chinese rule – was among the first in Lhasa. Amid the chaos, police ordered traders in the market to go home and soldiers were drafted in. The action was futile as protests began in other monasteries in support of the 15 monks and lay people began marching in support of Tibetan rights.

The monks – who were visiting Lhasa's Sera monastery – have not been seen since their arrest. Nothing is known of their condition or whereabouts.

With the province "locked down" by the police and army, and all foreign journalists and observers forbidden from travelling to Tibet, there is little firm information about the extent of the uprising. But unconfirmed reports suggest there have been more than 1,000 arrests in the province and about 100 deaths in clashes between Tibetans and the authorities.

Many other groups of monks have taken to the streets complaining that the authorities were increasingly restricting their religious freedoms. They were soon joined by groups of civilians protesting that their Tibetan identity was being eroded by a deliberately policy of flooding the area with the minority Han Chinese ethnic group.

The protests erupted into rioting four days later which Tibet's exiled government said claimed 80 lives.

Beijing appears to have quelled the unrest for the moment by sending troops to Tibet and the provinces of Gansu, Qinghai and Sichuan. But pressure is mounting on China to begin talks with the Dalai Lama, whom it has blamed for inciting the unrest. A group of 29 Chinese dissidents have signed an open letter calling for talks with Tibet's spiritual leader and demanding a UN investigation into the situation. Support is also growing for a boycott of the Olympics if Beijing persists in its brutal treatment of dissent.