China is ordering 100,000 nomadic Tibetans to move from grasslands into towns and villages in a bid to protect the environment, state media said.
The Tibetans are being relocated to protect the source area of key Chinese rivers in north-west China's Qinghai province, Xinhua news agency reported.
The grasslands are suffering from overgrazing, desertification and the effects of climate change, it said.
Some 60,000 people will be moved by year-end and 40,000 more by 2010.
Housing will be provided for all those forced to move, Xinhua said, but the head of one Tibetan community said it was not an easy process.
Qinghai province is a source for both the Yangtze and Yellow rivers, but experts have warned of a crisis over water supply.
Climate change is melting glaciers that feed the rivers and subterranean water supplies have been reduced by increased population and industrialisation, experts say.
The relocation programme, aimed at restoring the ecology of the grasslands, is China's biggest resettlement project, Xinhua said.
Those who move will be given accommodation and greenhouses in which to grow vegetables, Li Xiaonan, an official in charge of the project, told the agency.
But the BBC's James Reynolds, in Beijing, says that it appears that the nomads have no choice in the matter.
The ruling communist party says that everyone affected is being offered compensation, but that is unlikely to satisfy Tibetans, our correspondent says.
Many argue that China has been determined for many years to destroy their way of life as a people.
Environmentalists suggest that if China is really keen on protecting its environment it should focus its efforts on cleaning up its polluted rivers and reducing its carbon emissions, our correspondent says.