In the first reported account of the event, Jamyang Samten, 15, said he was among those who ended up being given electric shocks and forced to dig ditches "as a warning to others".

Samten was in a 75-strong group making their way over the 5,800-metre-high (19,000ft) Nangpa La pass in September when Chinese guards opened fire. At least two people, including a 17-year-old Buddhist nun, were killed.

The incident was filmed by a Romanian television producer on a mountaineering expedition, sparking an international outcry. Beijing had claimed the refugees were shot when border guards were attacked.

Of the survivors, 41 managed to reach India but 32 were caught and detained. The teenager said he was interrogated over a three-day period during which he was repeatedly hit with an electric cattle prod. "It went on until I fainted," Samten told reporters, adding that police repeatedly asked him to identify the dead nun.

After three days the Tibetans were taken to a prison in Shigatse, Tibet's second-largest city, Samten said. They were questioned again while chained to a wall, he said. "A guard wearing a metal glove would hit us in the stomach."

Samten was held in a labour camp there for 48 days and forced to dig ditches, build fences and work on fields, he said. Once released he paid guides to take him via Nepal to India where the Dalai Lama, the exiled Tibetan leader, has been based since 1959. "He wanted to come to see his holiness [the Dalai Lama] and he also wanted an education in the Tibetan language," said Tsering Ngodup, who works with the Tibetan refugee centre in Dharamsala.

The account could not be verified but echoes the stories of others who have made similar treks. Lobsang Gyaltsen, who managed to escape when Samten was captured, said he feared for his family in Tibet. "I do not know if they are safe. We come here to learn about our language and culture. These things are hard in Tibet where we do not have freedom."

Gyaltsen said he had been walking for 17 days when Chinese guards caught up with his group, who had eaten little and spent days struggling over ice and rock.

More than 4,000 Tibetans flee to Nepal each year across the border, which crosses several of the highest mountains on earth, including Everest. Supporters of the Dalai Lama claim China runs a repressive police state in Tibet. Beijing regards him as leading a separatist movement from exile.

With a new railway linking mainland China to Lhasa, Tibet's capital, Beijing has tightened its grip on the region. "China has become emboldened over Tibet," said Phunchok Stobdan, an Indo-Tibetan affairs expert. "Nobody dares to raise the issue with them."