China has opened the world's highest railway, celebrating the link into Tibet while critics denounce it as a threat to the Tibetan people's culture and environment.

A proud but sombre President Hu Jintao waved farewell as the first train left Golmud, the outpost in the far-western province of Qinghai where the new 1,142-kilometre track to the Tibetan capital, Lhasa, begins.

"The building of the Qinghai-Tibet railway is of major significance for accelerating the economic and social development of Tibet and Qinghai, improving the lives of people of every ethnicity, and strengthening unity between ethnic groups," Mr Hu told a meeting broadcast on Chinese television.

After his speech, a train carrying officials and model workers on the project set out from Golmud for Lhasa where it was due to arrive on Saturday night, Xinhua news agency reported. Another train set out from Lhasa.

The inaugural service from Beijing leaves on this evening, amid a crescendo of publicity, and reaches Lhasa 48 hours later, after a 4,000 kilometre journey touching altitudes of over 5,000 metres on the Tibetan plateau.

Critics fear the railway will spur an influx of tourists and migrants who will erode Tibet's cultural identity. China says Tibet is an inseparable part of its territory.

"Tibet is a part of China. If any Chinese want to go there, that is their choice," said Chinese Foreign Ministry spokeswoman Jiang Yu.

China's army occupied the Himalayan territory in 1950 to impose Communist rule.

Nine years later Tibet's chief spiritual leader, the Dalai Lama, fled to India after a failed uprising against Chinese rule.

'Black day'

Tibetans in Dharamsala in northern India, where the Dalai Lama presides over a government in exile, called Saturday a "black day" for Tibet. They have launched a website, www.rejecttherailway.com, in protest.

On Friday, three overseas activists of Students for a Free Tibet unfurled a banner at Beijing's main railway station reading "China's Tibet Railway: Designed to Destroy". Police quickly detained them.

China hopes the railway will boost Tibet's economy and reduce transport costs.

According to Chinese statistics, Tibet's average economic growth from 2001 to 2005 was more than 12 per cent a year, driven by injections of central government funds.

But critics say too little of that development benefits Tibetans who, with Chinese migrants already flooding in, are becoming an underclass excluded from prosperity.

"We're already seeing the marginalisation of Tibetans, and the railroad is the final achievement," said Kate Saunders of the International Campaign for Tibet.

"The railroad is a top-down project that prioritises the development of the military and the administrative state," she said.

Opponents also say the railroad, which crosses fragile, frozen highlands, is an environmental peril.

The Government counters that it has gone to unprecedented lengths to protect the environment, from carriages equipped with garbage compacters and vacuum toilets to special crossings for endangered Tibetan antelopes.

-Reuters