Hundreds of Khmer Rouge troops have fled Cambodia after being crushed by Vietnamese-led rebel forces.
The capital, Phnom Penh, has been seized and Pol Pot and many of his soldiers forced to retreat into the countryside.
It signals the end of nearly four years of brutal domination by the guerrillas.
Defeated soldiers crossed the border into Thailand where they were taken to prison as illegal immigrants.
The Thai authorities have said they will not be forcibly returned to Cambodia.
Thousands more are reported to be seeking refuge in an enclave of North West Cambodia.
The retreat is a serious blow for the Khmer Rouge, which having itself captured Phnom Penh four years ago began torturing and killing its opponents.
Fall of Phnom Penh is critical
An intelligence source said there was little evidence of an organised defence between the Cambodian capital and the Thai border – and that Phnom Penh was taken almost without a shot being fired.
It has been suggested that the Khmer Rouge decided to pull out of positions around the capital when they realised they would have little chance at beating the superior armed opposition.
The Vietnamese invasion came after a fierce year-long border war between Vietnam and Cambodia.
Vietnam launched its offensive on Christmas Day, with the help of pro-Vietnamese rebels in Cambodia assembled a few weeks earlier under the banner of the Front for National Salvation.
The fall of Phnom Penh is regarded by analysts as critical to the power balance in South East Asia.
Russia has firmly supported Vietnam while China has backed Cambodia.
Courtesy BBC News
In context
The Khmer Rouge’s reign of terror began in 1975 in the aftermath of the American bombing of Cambodia, which had increased the guerrillas’ initial popular support.
For the next four years Leader Pol Pot oversaw the deaths of an estimated 1.7 million people, by execution, forced labour and starvation.
In 1979, Pol Pot and the Khmer Rouge were forced from power by the invading Vietnamese.
Pol Pot officially resigned the leadership in 1985 but appears to have retained considerable influence.
Pol Pot was convicted of treason by a “people’s tribunal” in 1997, and was sentenced to life under house arrest.
He died in April 1998.
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