Although technically his trial began in February, when judges began hearing procedural issues, Monday marked the start of the substantive phase of the trial which will see witnesses called to give testimony and give Duch the chance to publically tell his story.
Duch is described as the chief torturer for the Khmer Rouge, heading the S-21 Tuol Sleng prison, the regime's largest interrogation facility, where at least 14,000 men, women and children were tortured.
He is the first of five senior Khmer Rouge members to go on trial and faces life imprisonment if convicted.
Duch has expressed regret for his actions and asked for forgiveness from his victims, but maintains that he was only following orders from his superiors.
After the fall of the Khmer Rouge thousands of documents were recovered from S-21 recording the treatment of prisoners.
One shows Duch's signature on a list of detainees, with the words "Kill them all".
After the Khmer Rouge fell from power Duch disappeared, living under two other names, returning to his previous job as a teacher and converting to Christianity before he was discovered by chance by a British journalist in the Cambodian countryside in 1999.
Since then he has been in detention awaiting trial, amid years of political and procedural wrangling.
|The Khmer Rouge oversaw the deaths of about 1.7 million people [GALLO/GETTY]
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.
Duch's trial is a momentous event in a country where about a fifth of the country's population died from starvation, disease or execution during the Khmer Rouge's rule.
His testimony is also expected to be vital in securing the convictions of the four other Khmer Rouge leaders awaiting trial before the tribunal.
The group includes Nuon Chea, the 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 crimes.
Pol Pot, the group's former supreme leader and so-called Brother Number One, died in a jungle hideout in 1998.