"Your honours must objectively, we say, review the evidence to determine whether or not what has been accepted by the accused amounts to full disclosure and the full truth," lawyer Karim Khan told judges.
"In large and important material particulars, even today, the accused has sought to evade or minimise his role and the awful reality that was S-21 [prison] and the regime that operated there and the fate and suffering that befell so many civil parties that we all represent."
The prosecution is scheduled to begin presenting its arguments on Tuesday and proceedings are due to conclude on Friday.
Pleading for forgiveness
Since his trial began in February, Duch, 67, has repeatedly asked for forgiveness for the deaths at S-21 prison, a former high school also called Tuol Sleng.
He is charged with crimes against humanity, war crimes, torture and premeditated murder, and faces a maximum term of life in prison by the tribunal, which does not have the power to impose the death penalty.
A verdict is expected early next year.
Hundreds of Cambodians attended the specially built courtroom on the outskirts of Phnom Penh on Monday, watching Duch, who sat behind a huge bulletproof screen to prevent possible revenge attacks.
|Hundreds of Cambodians have
attended the trial [AFP]
This week's proceedings will be broadcast live by all Cambodian television stations, court officials said.
Tuol Sleng prison was at the heart of the Khmer Rouge security apparatus and inmates were taken from there during Duch's tenure for execution at nearby Choeung Ek, an orchard now known as the "Killing Fields".
Led by "Brother Number One" Pol Pot, the Khmer Rouge was responsible for one of the worst horrors of the 20th century.
Rising to power as a tragic spin-off from the US conflict in Vietnam, the movement emptied Cambodia's cities to take society back to a rural "Year Zero," purging city dwellers and intellectuals.
The Khmer Rouge was ousted by Vietnamese-backed forces after a reign of terror lasting almost four years, but continued to fight a civil war until 1998. Pol Pot died in the same year.
Duch has been detained since 1999, when he was found working as a Christian aid worker in the jungle, and was formally arrested by the tribunal in July 2007.
The court has faced controversy over allegations of interference by the government and claims that Cambodian staff paid bribes for their positions at the court.
The joint trial of four other more senior Khmer Rouge leaders is expected to start in 2011.
The court is also investigating whether to open more cases against five other former Khmer Rouge cadres after a dispute between the international and Cambodian co-prosecutors over whether to pursue more suspects.