Duch's testimony comes after he formally apologised last week for the deaths of more than 14,000 people at the S-21 torture and interrogation centre he ran in Phnom Penh.
"I joined the revolution in order to transform society, to oppose the government, to oppose torture"
Kaing Guek Eav, AKA Duch
The 66-year-old former teacher is charged with crimes against humanity and war crimes.
Although the tribunal's mandate covers only the Khmer Rouge's time in power, Duch's testimonies later this week are also expected to cover an earlier period before the Khmer Rouge takeover when he commanded a secret jail called M-13.
Nil Nonn, a judge, said it was necessary to hear about M-13 to understand Tuol Sleng's organisational structure, Duch's personality, and the relevance of his role to the Khmer Rouge leadership.
Jean-Marc Lavergne, a French judge, questioned Duch about his past, particularly how he came to be sentenced to 20 years in prison for communist activities against the government of Prince Norodom Sihanouk in the 1960s.
"I joined the revolution in order to transform society, to oppose the government, to oppose torture," Duch said.
Duch recounted his stay from 1968 to 1969 in Prey Sar prison - a facility which he would later use as part of his Tuol Sleng complex.
"That was when prisoners were shot and terrorised for every breath they took," Duch said of his time in the military prison.
US policies blamed
Despite Duch's dedication to communist ideology, he said that he realised that the Khmer Rouge's activities would end up in a "disaster".
He also said that US policies in the 1970s contributed to the communist regime's rise to power.
Duch said he believed the Khmer Rouge movement would have died out by 1970 if the US government under Richard Nixon had not supported Cambodia's military-led government following the 1970 coup d'etat that removed Prince Norodom Sihanouk from power.
"I think the Khmer Rouge would already have been demolished," he said of their status by 1970.
Instead, he said, the Khmer Rouge took what they saw as a "golden opportunity" that was given by the backing given to General Lon Nol, the coup leader, by Nixon and his secretary of state, Henry Kissinger.
Because of this alliance, the Khmer Rouge were able to build up their power over the course of their 1970-75 war against the Lon Nol regime, Duch said.
Critics of US foreign policy at the time say the US agreed to the coup because Washington felt Sihanouk's "neutralist" policies benefited the communists in Vietnam, who used Cambodian territory as a base and supply line.
But the coup triggered a greater Vietnamese communist presence in Cambodia and caused them to vastly increase their support for the Khmer Rouge.
|Duch told the tribunal he knew that Khmer Rouge policies would end in "disaster" [AFP]
Duch is the first of five former Khmer Rouge officials expected to face trial before the UN-Cambodian tribunal.
But with the first trial only just beginning, allegations of corruption are threatening to undermine the tribunal process.
On Monday Peter Taksoe-Jensen, the UN assistant secretary-general for legal affairs, began meetings with government and tribunal officials about allegations that Cambodian personnel taking part in the tribunal were forced to pay kickbacks to obtain their positions.
Defence lawyers and human rights groups have said the allegations, if unanswered, could tarnish the tribunal's credibility.
The allegations also pose a financial threat, as foreign aid donors who provide the budget for Cambodian personnel are withholding their funds until the issue is resolved.