Khmer Rouge leader Pol Pot's former torturer has gone on trial for crimes against humanity.

Comrade Duch, who is the first of the top Khmer Rouge cadres to face trial, ran the Cambodian regime's most notorious S-21 torture camp in the capital Phnom Penh where at least 14,000 men, women and children were murdered.

The joint Cambodian-United Nations tribunal was set up to prosecute "those most responsible" for one of the darkest chapters of the 20th century.

Nearly two million people were executed or died of starvation during the 1975-79 Khmer Rouge era.

Also known as Kaing Guek Eav, he did not address the court, but sat impassively as lawyers haggled over procedural matters.

Duch also faces charges of war crimes, torture and homicide while chief of S-21 where prisoners were tortured to confess to crimes they did not commit before being shot or beaten to death in the "Killing Fields" outside the capital.

Hundreds of Khmer Rouge victims, including saffron-robed Buddhist monks, packed the public gallery, reacting with anger and relief at the sight of 66-year-old in the dock.

"This is the day we have waited for for 30 years," said Vann Nath, 63, one of only 12 survivors from S-21 where "enemies of the revolution" were tortured and killed.

"Duch's hands are full of blood. It's time for Duch to pay for his actions," said 39-year-old Norng Chan Phal, a child survivor of S-21 who is expected to give evidence at the trial.

This week's hearings will lay the groundwork for a full-blown trial in March, when Duch and survivors are expected to testify.

The trial is a landmark for the country where nearly everyone lost loved ones in the violence, starvation and disease that followed Pol Pot's dream of an agrarian utopia.

It also ends a decade of wrangling over jurisdiction and cash at the tribunal, which had left many Cambodians wondering if Pol Pot's surviving henchmen would ever face a judge.

Duch is one of five ageing senior cadres charged for their roles in the "Year Zero" revolution that ended when a Vietnamese invasion forced the Khmer Rouge back into the jungles in 1979.

He is expected to be a key witness in the trials of "Brother Number Two" Nuon Chea, Khieu Samphan, who was the regime's ex-president, and Ieng Sary, its foreign minister, and his wife.

These four have denied knowledge of any atrocities by the Khmer Rouge during its rule, which began by driving everyone out of the cities with whatever they could carry to begin a life of slave labour in the countryside.

If convicted, the five face life in prison.

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