BBC Correspondent David Jameson: An amnesty deal may have been struck with the government

Khieu Samphan was the group's nominal leader in the 1980s, while Nuon Chea was a shadowy figure who devised much of the Khmer Rouge's extreme left-wing ideology.

In a letter to the government, they said they wanted to end their days as simple citizens.

But there are growing international calls for Khmer Rouge leaders to be tried for crimes against humanity.

Simon Ingram reports from Bangkok: "We are seeing the death throes of the Khmer Rouge"

Cambodian Government officials said the pair surrendered in Pailin, a town close to the Thai border which was once a stronghold of the Khmer Rouge.

Their capture means the only senior Khmer Rouge commander still on the loose is Ta Mok, a lieutenant of Pol Pot, the notorious Khmer Rouge leader who died of a heart attack in April.

Ta Mok, known as The Butcher, is believed to be in hiding on the Thai border with a handful of loyal supporters.

Unconditional surrender

The two men reportedly surrendered unconditionally after sending a letter to Cambodian Prime Minister Hun Sen. They said they hoped their defection would "strengthen peace and stability, national reconciliation and the development of Cambodia".

Hun Sen, himself a former Khmer Rouge field commander, responded in a letter to King Sihanouk, who has endorsed the move.

The Prime Minister said: "I would like to warmly welcome the two personalities and hope this decision will become a mobilisation of strength for national reconstruction and peace."

Khieu Samphan: Public face of the Khmer Rouge

Khieu Samphan became known as the public face of the secretive Khmer Rouge when he represented the movement during the 1991 Paris peace talks.

A doctoral thesis he wrote in Paris in the 1950s, which urged a return to agrarianism, is seen as the blueprint for the Khmer Rouge's disastrous policies of the late 1970s.

Khieu Samphan was officially head of state in Cambodia between April 1976 and the overthrow of the guerrillas by the neighbouring Vietnamese in 1979.

Death toll

In that period - later dubbed the Killing Fields after the Oscar-winning film - an estimated 1.7 million Cambodians died through disease, starvation or execution.

Nuon Chea, now 71, was the official number two to Pol Pot, who died in a small village in northern Cambodia.

He was president of the Cambodian parliament between 1976 and 1979 and remained a top leader throughout the group's period in the wilderness following the Vietnamese invasion.

The pair's reported surrender comes two years after the defection of top Khmer Rouge cadre Ieng Sary, which coincided with a fatal split in the movement.

There have been widespread international calls for Khieu Samphan, Nuon Chea and Ta Mok to go on trial.

The last Khmer Rouge fighters surrendered to the government earlier this month.