Kaing Guek Eav, better known as Duch, is the first Khmer Rouge cadre to face justice in an international tribunal over the deaths of up to two million people through starvation, overwork and execution under the 1975-1979 regime.

But to the dismay of survivors and relatives of victims, the court took into account the years he had already served since his arrest in 1999, meaning that the 67-year-old could walk free in about 19 years.

Duch apologised during his trial in Phnom Penh for overseeing mass murder of 15 000 men, women and children at Tuol Sleng prison, also known as S-21, but shocked the court in November by finally asking to be acquitted.

Crowds of Cambodians, including regime survivors and Buddhist monks, turned up at the specially built court on the outskirts of Phnom Penh, hoping finally to see justice for the Khmer Rouge's crimes. Many left disappointed.

"Millions of people were killed, millions of dollars have been spent [on the trial], but the killer could be released," said a tearful Chum Mey (79), one of only a handful who survived imprisonment and torture at the jail.”

We can't accept a sentence where it is conceivable that he could walk even for one minute in society," said Theary Seng, founder of the Cambodian Centre for Justice and Reconciliation, whose parents were killed by the Khmer Rouge.

Duch was initially given 35 years but the court reduced the jail sentence after ruling that he had been detained illegally for years by a Cambodian military court before the UN-backed tribunal was established.

Prosecutors, who had asked for a 40-year prison term, hailed the judgement as an historic day for Cambodia, although they said they would consider appealing for a stiffer sentence.

The joint trial of four more senior Khmer Rouge leaders charged with genocide is expected to start in 2011.