THERE is an extraordinary scene in When the Dragon Swallowed the Sun (Cert. 15) in which the 18th descendant of the Great Religious Kings of Tibet receives a prophecy from the State Oracle. Dressed in ornate ceremonial robes and an enormous fur-topped headress, as if encased in a giant bell, the medium of the oracle dances, possessed, amid the undulating chanting of the gathered monks. Finally, he squeaks his divinations, scribbled down frantically by an attendant.

It's a fascinating insight into the ancient culture of Tibet, the treasure that many of the film's subjects are desperately trying to preserve, mostly from their exile in India. The threat is not just external - thousands of Chinese people are arriving in Tibet under the policy of population transfer - but internal: young Tibetans express a desire to focus not on a decades-long struggle for liberation but a successful professional career.

The film, seven years in the making, was clearly a labour of love for director Dirk Simon. He has collected interviews from a huge number of sources, including the Dalai Lama, and activists from across the spectrum.

At the heart of his film is a tension between those still committed to a free Tibet, independent of China, and those who have accepted the Middle Way, the approached proposed by the Dalai Lama, which calls for autonomy rather than independence. All Tibetans are divided in two, one interviewee explains, between loyalty to their spiritual leader, and their "right to freedom".

Perhaps the most passionate contributor is Lhasang Tsering, an activist who joined the armed Tibetan resistance force. He speaks of recruiting a force of "mosquitos" to attack China's industry, cutting wires and causing blackouts: "The price of freedom is not paid in gold and silver, but in flesh and blood," he insists, desperation etched on his face.

What is striking is the contrast between the weariness of those calling for the Middle Way, who point to China's strength, and the passion of those who still believe in the original struggle. For these more radical campaigners, the stories of their parents has been a crucial influence. "I am ready to sacrifice my body any time," says one young man. This is not empty rhetoric. In March, 26-year-old Jamphel Yeshi died after committing self-immolation in New Delhi.

A thread running through the film is the build-up to the Beijing Olympics of 2008, including a campaign to prevent the torch from passing through Tibet and a daring protest on the Golden Gate Bridge. During clashes on the streets of San Francisco, operatic music from Philip Glass reaches an almost deafening crescendo as protesters confront pro-China opponents, faces within spitting distance of one another.

Although participants in the film speak of the need to understand China, there is little explanation of China's perspective, beyond these clashes, in which Chinese immigrants insist that Tibet is part of China. There is also an interview with a Chinese dissident artist who describes how his countrymen are "brainwashed", taught that the Red Army freed Tibetan slaves from a regime under which human skin was used to make furniture.

The maker of the film is clearly enthralled by his subject-matter. There are stunning shots of Tibet, clouds scudding over rolling hills, the sun rising over dancers on hilltops, their costumes silhouetted against the sky. But the thoroughness of his investigation into the complexities of the situation precludes any suggestion of his exoticising the mysterious East.

Towards the conclusion of the film, Jigme Yeshi of the Tibetan Youth Congress seeks to revive the history of Tibet's warriors, submerged, he suggests, in recent narratives that emphasise peace. His speech is played over footage of young Tibetans competing in the amateur Tibetan Olympics in India. It is a poignant juxtaposition: the passion of a young freedom fighter pitted against pictures of his peers wielding not arms but javelins, young men and women born to an exiled people numbering just six million, while over the border a dragon of 1.3 billion people roars.