Duch, 65, is accused of murder, torture, rape and persecution, and faces life imprisonment if convicted. Cambodia does not have a death sentence.
Duch has admitted atrocities were carried out, but has said he was only following orders from superiors.
A former maths teacher, he headed the S-21 prison, the largest Khmer Rouge interrogation facility, where at least 14,000 men, women and children were detained and tortured.
Today the prison, a former high school, has been converted into a museum of the atrocities committed by the Khmer Rouge.
Many of the centre's victims were executed in what later became known as the "killing fields".
On Tuesday more than 500 people packed into the courtroom to watch the initial court hearing, among them Vann Nath, one of less than 20 known survivors of S-21.
"It is not only me wanting justice today. All Cambodian people have been
waiting for 30 years now," he told the Associated Press.
|Hundreds of survivors of the Khmer Rouge watched the trial begin [AFP]
"I look at Duch today and he seems like an old, very gentle man. It was much different 30 years ago."
Vann Nath survived after Khmer Rouge guards discovered he could paint and sculpt busts of Khmer Rouge leader Pol Pot. He described Duch as a "very cruel man".
Francois Roux, his lawyer, told Reuters TV that Duch had asked victims for forgiveness on the eve of the proceedings.
A handful of S-21 survivors were expected to attend Tuesday's hearing.
Duch's testimony is expected to be vital in securing the convictions of four other accused Khmer Rouge leaders held awaiting trial before the tribunal.
They include Nuon Chea, the group's former deputy leader also known as "Brother Number Two"; Khieu Samphan, the former Khmer Rouge head of state; Ieng Sary, the former foreign minister; and his wife Ieng Thirith, who was the social affairs minister.
All have denied committing any crimes.
Pol Pot, the group's former supreme leader and "Brother Number One", died in a jungle hideout in 1998.
After the fall of the Khmer Rouge in 1979, Duch disappeared for 20 years living under aliases and converting to Christianity before he was located in northwestern Cambodia by a British journalist in 1999.
William Smith, one of the prosecutors, said the initial stage of the hearing beginning on Tuesday was expected to last one or two days and will involve procedural matters that will "decide the shape and structure of the trial".
He said the judges will then finalise the witness list and decide on preliminary legal objections, adding that full testimony was not expected until March.
|Ordinary Cambodians have awaited justice over the 'killing fields' for decades [Reuters]
The joint war crimes tribunal, established in 2006 after nearly a decade of negotiations between Cambodia and the UN, is seen as the final chance for Cambodians to see justice done for the Khmer Rouge's atrocities.
But the run-up to the trial has been marred by claims that the government is interfering to stop the prosecution of further suspects, as well as allegations that Cambodian court staff were paid kickbacks for their jobs.
"Any hint of political manipulation at the tribunal will undermine its credibility with the Cambodian people," Sara Colm, a Cambodia-based senior researcher at Human Rights Watch, said.
"Until allegations of corruption and improper interference by the government are investigated and resolved, the tribunal’s integrity as a legitimate and independent court will remain in question."
But for ordinary Cambodians, the trial remains their only chance to heal the wounds of a traumatic period that saw almost a quarter of the population wiped out.