|Duch's archive of photos and confessions helped trace the final months of thousands of inmates' lives [EPA]
Cambodian lawyers for Duch, a former Khmer Rouge prison chief, have begun an appeal against his 35-year jail sentence for war crimes and crimes against humanity.
The 68-year-old, whose real name is Kaing Guek Eav, has admitted to overseeing the deaths and torture of around 15,000 men, women and children at the Tuol Sleng prison in Cambodia's capital during the communist regime's 1975-1979 rule.
Duch, who returned to the courtroom on Monday, was sentenced last July to 35 years in prison for war crimes and crimes against humanity.
Al Jazeera's Stephanie Scawen, reporting from outside the court in Phnom Penh, said the sentence was commuted to 19 years due to time already served and on the grounds that Duch had been illegally detained for years.
"This was reduced by five years for his illegal detention," she said.
"And [he was] then given credit of 11 years for time already served. Some say that means he only had a 30 year sentence but the court this morning did repeat it was 35 years."
Prosecutors are seeking a harsher sentence against Duch. But his lawyers are calling for his release, saying he was only following orders. They argue that the case falls outside the court's jurisdiction.
"The case centres on whether Duch was a senior leader or one of those most responsible. Lawyers say documents show he was a low ranking cadre and therefore not a leader," our correspondent added.
"He had to follow orders and did not act alone, he had no power to decide fate of prisoners including that of friends and family - even his own brother-in-law."
During his trial in Cambodia's UN-backed war crimes court, those who worked under Duch at the detention centre had testified that he was universally feared.
Duch apologised during the trial for crimes committed under his command at the jail, where prisoners were tortured into denouncing themselves and others as foreign spies.
But, victims questioned whether his remorse was genuine after he asked to be acquitted in his closing remarks.
Anne Heindel, a legal adviser to the Documentation Centre of Cambodia, which collects evidence of Khmer Rouge atrocities, said Duch did not accept full responsibility for the crimes.
"He believes he was a cog in the communist party wheel, that because he was not fully autonomous in his decision-making, his guilt is lessened," she said.
Born in 1942 in central Cambodia, Duch is remembered as a sincere teacher devoted to helping the poor before he became a Khmer Rouge cadre in 1970.
"I joined the revolution in order to transform society, to oppose the government, to oppose torture," he said during his trial.
"I sacrificed everything for the revolution, sincerely and absolutely."
Inside the rebel-controlled zones, he chose Duch as his revolutionary name because it was used by a model student in a schoolbook from his youth.
He oversaw a series of jungle prisons before being made head of Tuol Sleng after the regime seized Phnom Penh in 1975.
What began with as a few dozen prisoners turned into a daily torrent of condemned coming through Tuol Sleng, or S-21, as the regime purged itself of its "enemies".
Ever meticulous, Duch built up a huge archive of photos, confessions and other documents with which prosecutors traced the final months of thousands of inmates' lives.
Following the Khmer Rouge's fall from power, he maintained posts within the communist movement as it battled Vietnam-backed troops.
He was arrested after Nic Dunlop, an Irish photojournalist, uncovered him working for a Christian aid agency in western Cambodia under a false name.
Before that, many had long assumed he was dead following his disappearance after Vietnamese troops ousted the Khmer Rouge in 1979.
"I told Nic Dunlop, 'Christ brought you to meet me'," Duch told his trial.