A former Khmer Rouge prison chief who oversaw the torture and killings of 17,000 people was charged with crimes against humanity by Cambodia’s UN-backed tribunal today, in the most decisive step yet towards bringing those responsible for the country’s genocide to justice.

Duch, whose real name is Kang Kek Ieu, was transferred into custody of the tribunal, becoming the first Khmer Rouge figure to be charged in a long-stalled judicial process. He had been held at a military prison since his arrest in 1999, some 20 years after the fall of the Khmer Rouge.

Tribunal judges spent several hours interviewing Duch before formally filing charges against him.

"The co-investigating judges of the Extraordinary Chambers in the Courts of Cambodia have charged Kang Kek Ieu, alias Duch, for crimes against humanity and have placed him in provisional detention," they said in a statement.

The first inmate at the tribunal’s newly-built detention centre, Duch is one of five Khmer Rouge leaders under investigation by the tribunal. However many, including Brother Number One Pol Pot, will never face justice, having died before the tribunal was established.

As chief of the notorious Tuol Sleng prison in Phnom Penh – known to the Khmer Rouge as S21 – Duch presided over the incarceration, torture and finally killings of an estimated 17,000 so-called enemies of the Pol Pot regime. Of the inmates – including women, children, and babies born in the prison – there were only eight known survivors, who later recounted their sickening ordeals.

One, Cambodian artist Vann Nath, who has depicted his time in Tuol Sleng in numerous paintings, said today he was reluctant to confront his former tormentor. “I don’t want to confront him unless the tribunal wants me to do so,” he said.

Many of S21’s prisoners were former Khmer Rouge members and their families whose loyalties came under suspicion from an increasingly paranoid leadership. After being subjected to months of interrogation and brutality - survivors tell of being electrocuted, beaten, burnt with searing metal and forced to eat human faeces – those who had not died from the abuses were taken to the Choeung Ek Extermination Centre on the outskirts of the city, now known as the Killing Fields. There they were beaten to death with iron bars and machetes – bullets were deemed too precious for such purposes – before being thrown into mass graves. One of the execution orders signed by Duch was for 17 children who failed to inform the party of their parents' alleged treachery.

Blood still stains the floors at the prison – chillingly, a former high school, where the gym bars in the courtyard were turned into gallows. Now the Tuol Sleng genocide museum, instruments of torture and victims’ skulls are displayed alongside photographs of every prisoner admitted. Extensive records kept by Duch and other staff detail the prisoners’ experiences, while photographs taken by the Vietnamese on capturing the capital city show the prison as it was left by fleeing S21 guards, with mutilated bodies still chained to rusting metal bedframes.

But Duch’s lawyer, Kar Savuth, said the prison chief - who taught mathematics before becoming a communist revolutionary in the late 1960s - was not guilty of any crimes and was only following “verbal orders from the top."

“He had no rights to arrest or kill anyone,” Kar Savuth said.

An estimated two million people died during the four years of Khmer Rouge rule, from starvation, overwork or execution. Upon seizing power in 1975, the ultra-communists emptied Phnom Penh and other urban centres, forcing their populations into rural labour camps. The regime abolished religion, currency and schools, turning Cambodia back to “Year Zero” in its bid to create an agrarian utopia. Instead, it perpetrated what is now regarded as one of the worst atrocities of the 20th century.

Its crimes were part of a “common criminal plan constituting a systematic and unlawful denial of basic rights,” prosecutors said earlier this month after submitting their cases for investigation.

Attempts to bring the perpetrators to justice have been frustrated by years of wrangling between Cambodian authorities and the United Nations over the trial process. It was only in 2003 that a tribunal plan was finally agreed and set in motion, by which time many of the accused, including Pol Pot, had died. The process now underway is widely regarded as the last opportunity to secure justice for the Cambodian people.

While the names of all those under investigation have not been made public, prosecutors are reportedly also seeking charges of genocide and other crimes against former Khmer Rouge head of state Khieu Samphan, as well as Pol Pot’s deputy Nuon Chea and foreign minister Ieng Sary.

Khieu Samphan said today he was not alarmed by Duch’s summons, saying his own possible appearance at the tribunal “won’t be a problem at all."

“There is no reason to arrest me. I will go to the tribunal if they ask me to,” he said from his home in northwestern Cambodia.

“I have my lawyer and am prepared,” he added.

As for Duch, his present feelings on the tribunal and his alleged crimes are unknown. But in a transcript of a government interview in 1999, he claimed he was not a “cruel man” and was instead “an individual with gentle heart caring for justice ... since childhood.”

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