Diamond warrior: Roy Sesana, Spokesman for The Kalahari Bushmen

Evicted from their ancestral lands in the Kalahari when gems were discovered there, Botswana's bushmen are now trying to enlist Leonardo DiCaprio's help in their fight to return home. Their leader talks to Katy Guest

Standing in a darkening autumn street in central London, Roy Sesana cuts a noble and lonely figure. He wears a beaded amulet on his head to offer the protection of his ancestors as he travels, and his yellow T-shirt brings a splash of African colour to the dim evening: it is obvious that he is not at home.

But Sesana, now in his mid-sixties, is in London because he no longer has a home. In 1986, the Botswanan government announced its decision to move him and his people from their land, after diamonds were found on the Central Kalahari Game Reserve (CKGR). De Beers quickly bought the rights to mine there, and since 1997, most of the 5,000 bushmen have been living in camps. Now, through Mr Sesana, they have askedLeonardo DiCaprio to help publicise their plight.

"Your film shows how badly diamonds can hurt," Mr Sesana has told DiCaprio, who stars in the forthcoming film Blood Diamond. "We know this. This is the land where we were born, where our ancestors are buried. We have lived there since the beginning of time. Now we are dying in the government relocation camps. We are full of pain."

The bushmen hope that the actor will use his influence as other unlikely supporters have done. Since discovering De Beers's links with Botswana, the models Lily Cole, Erin O'Connor and Iman have refused to work with the company. Gloria Steinem and Julie Christie are among celebrities who have supported the charity Survival International in protesting against the trade.

Sheila Khama, the CEO of De Beers in Botswana, says: "We are certain that our diamond mining activities were nothing to do with the removal of these people from the Kalahari" and insists that "De Beers complies with the laws of Botswana". But Mr Sesana is not the only one who wishes that they would do more than that.

The Gana and Gwi bushmen have lived in the CKGR for 20,000 years - although Mr Sesana says he can "only" count six generations back. In the 1980s, all this changed. Diamonds were found at a bushman community called Gope, and De Beers bought the right to mine there. When Botswana's British colonial rulers created the 20,386 square mile game reserve in 1961, the 5,000 bushmen who still lived there were given the right to remain in perpetuity. But soon that right started to be questioned. The Botswana government says it is worried about protecting the animals in the game reserve. But Mr Sesana explains how a single animal such as an antelope is shared between a family of 20 to 30 for a month. Every part is used, he says, and "nobody is selfish".

"After the first President of our country died in 1981, it was the second President who started the relocation," Mr Sesana remembers. "The day after his death the ministers came to tell us that we had to move from the CKGR because of diamonds and because it is a game reserve.

"I remember asking them, when we die, are you going to bring us and bury us where our forefathers are buried? They said no, that will not happen."

At the beginning, the bushmen refused to go anywhere - many of them even when the government cut off the borehole water supply that had existed since the creation of the reserve. Gradually, some took up the government's offer. "In 1997 the government tried not to persuade people to leave, but to trick people and buy people with money. They promised them new buildings."

The relocation camps have no game, wild foods or firewood, and those who followed official advice and moved there have to rely on government handouts. Many have fallen victim to TB, alcoholism, HIV and Aids. "We are dying in the government relocation camps," says Mr Sesana.

At the end of last year, an 81-year-old woman called Qoroxloo Duxee died of starvation, dehydration and shock at Metsiamenong. Mr Sesana asked for permission to be bury her beside her ancestors, but was forbidden.

"I tried to go there with water and food," he says. "I was punched, I fell down. I was jumped on by soldiers with boots. I was handcuffed and put in jail."

On the day he was released, he and his group were awarded the Right Livelihood Award, otherwise known as "the alternative Nobel Prize", for "resolute resistance against eviction from their ancestral lands and for upholding the right to their traditional way of life".

This month, their lawyers submitted their concluding arguments in Botswana's longest-running and most costly court case - the bushmen's case against the government, which began in July 2004. The result will be announced on 13 December, and Mr Sesana is optimistic.

"If there is justice," he says, "I think we will win. It would be like glory to go back." It may be a pyrrhic victory, though, as 10 per cent of the bushmen have died since the case began. Yet Mr Sesana will not give up. The day the bushmen step back on to their land, he says, "it will be like walking into the light".