Tash does not look like a man who has just put his life in danger. But as he sits in a cosy editing suite in London, the images on the screens around him – a Tibetan political prisoner showing his scars, a still of Tash interviewing a Buddhist monk – prove the contrary. He has risked his life at least twice: the first time, 11 years ago, to escape his native Tibet; and then, as the screens document, when he went back with a hidden camera to expose what he felt were injustices perpetrated by the Chinese government. "I can now never go back to Tibet," he says. "But it is worth it."

What makes his actions particularly dangerous is the Chinese government's blanket ban on journalists entering Tibet. His report for Channel 4's Dispatches reveals detail not seen before: reports last month of the recent uprisings could only be given by major news sources from vantage points outside the country – usually Nepal – conveying what snatches of second-hand experiences they could garner from the other side of the Himalayas. Tibet has an estimated one Chinese soldier for every 20 Tibetans – as opposed to one soldier per 1,400 Chinese citizens. This country, about the size of western Europe, has been firmly in the grip of the Chinese government since the Dalai Lama fled in 1959.

Tash fled Tibet, too, when he was 18, without telling his family. Yet as a boy he had been protected from knowing too much of the political repression. "I knew there were some people who had the Dalai Lama's book My Land and My People," he says, "but when I saw them talking they wouldn't let me join in – I was too young."

He says everybody practised in secret. "Boys would secretly watch the films of the Dalai Lama teachings, but no one knew anything of the outside world." Eager to escape to that unknown, Tash travelled the treacherous journey across the mountains to India, past frozen bodies half buried in the snow, to freedom.

Not everyone is so fortunate. Footage captured by Western climbers in September 2006 (and shown in the Dispatches programme) has a line of refugees plodding through the snow, with some of their number suddenly picked off by bullets fired by the Chinese soldiers behind them. "They shot a girl dead right in front of me and dumped her corpse in a hole nearby," one of the group remembers.

These people were deliberately escaping from what they considered Chinese tyrannies. As a young refugee looking for an education in India, though, Tash didn't realise the insulated nature of his old life until the political relevance of his new-found freedom began to hit home. "On Tibetan television almost every night, there would be stories about the Japanese invading China, committing genocide, beheading the Chinese and raping girls. I used to hate Japanese people, until I came into India and realised that it was propaganda," he recalls. The memory of a life in Tibet without fear seemed even more preposterous during his three-month undercover operation there last summer.

"When we were in Tibet I was greatly shocked," he says, clenching his hand into a gentle fist. "We're going to lose all Tibetan identity soon. In Lhasa, if you don't speak Chinese, it doesn't matter how good your Tibetan or English is, you don't get a job." And the fading of the ethnic way of life, he was distraught to find, is down to more than this systematic wearing away of cultural and religious ties. Through tip-offs and a web of contacts, he discovered that Tibetan women are being forcibly sterilised.

One woman agreed to speak to Tash, despite the cultural propriety that would rarely see a woman speak about such intimacies with a man, and the obvious dangers of criticising the government. "I was taken away against my will," she explains. She has two children – more than the "one child" policy allows – and could not afford to buy a certificate that stated she had been sterilised. "Apparently they cut the fallopian tubes and stitch them up," she says ruefully. "When they opened me up they pulled them out by the roots. It was agonisingly painful." They didn't use anaesthetic, or provide any drugs aside from aspirin. "I was sick and giddy," she says. "From the day after the operation I had to look after myself. If I needed a drip I had to pay for it myself."

Anyone who speaks out against the policies of the Chinese government like this, or calls for the freedom of Tibet, is in danger of being condemned a "splittist" – someone who is splitting from the Communist Party – and sent to prison. This, Tash discovered, can be for as little as raising a Tibetan flag in a meeting. A farmer, found guilty of this crime, explains: "I spent the prime of my life in prison ... from the age of 24 to 37." And so, the culture of fear is continually reinforced by harsh sentences for apparently minor crimes. An 18-year-old Buddhist monk, Tash says, was recently sent to prison for three years for inscribing "Free Tibet" in a book.

And time spent in a Chinese prison invariably means torture. One ex-political prisoner on Tash's film explains the use of handcuffs: "There are types that bind the two thumbs together," he says, demonstrating. "And others are serrated so they cut into the flesh of the wrists. They handcuff you and hang you from the ceiling then beat you. They strike your body with iron bars."

A Human Rights Watch report in 2007 claimed that tens of thousands of Tibetans have been moved into permanent camps. Tash visited a cluster of little concrete homes, miles away from any town: the people he spoke to expressed unhappiness, but with their livestock confiscated and roaming on the grasslands forbidden, they have no hope of changing things. Apart from protest, of course, but openly protesting against the police is widely acknowledged as a way to bring your life to a swift and bloody end.

Only after spending time in his homeland with the perspective of freedom does Tash understand an incident in his youth that he, blissfully ignorant, could not comprehend at the time. "When I was about 16 I sang an old song about the Dalai Lama at my village's New Year festival," he says. A friend had given him the words, and he didn't know it was banned. "When I sung, the old men and women were crying, I didn't know why. The head of the village thanked me and put a red scarf round my neck." Now he sees the situation all too clearly. "Tibetans," he says, "are trapped. They are like birds in a net."

'Dispatches: Undercover in Tibet' is on Channel 4 tomorrow at 8pm