A ruling on his release will be issued at a later date, judges said.
Arguing that Duch could flee if freed, co-prosecutor Robert Petit said that the UN-backed court had a duty to the regime's victims to keep him detained.
"There is sufficient evidence to support his responsibility for the crimes he's being charged with," Petit said.
"We believe that Duch's detention is necessary to ensure his presence in the court," he told AFP following the court's first public hearing, seen by many as a landmark moment for a country trying to come to terms with its brutal past.
Duch, whose real name is Kaing Guek Eav, allegedly oversaw the torture and extermination of an estimated 16,000 men, women and children at the Khmer Rouge's Tuol Sleng prison during the regime's 1975-1979 rule.
He was arrested by the tribunal in July, becoming the first top Khmer Rouge cadre to be detained and charged with crimes against humanity ahead of a trial expected to take place next year.
Sitting grim-faced before the panel of five judges who will rule on his release, Duch, a 65-year-old former maths teacher, appeared to be closely following the proceedings.
As judges ended the hearing, he rose clasping his hands in front of him and said: "I ask the pre-trial chamber to release me on bail."
Duch's bail hearing followed the arrest Monday of another regime figure, head of state Khieu Samphan, who was charged with war crimes and crimes against humanity.
The arrest brought to five the number of former top cadres facing justice for one of the 20th century's worst atrocities.
Up to two million people were executed or died of starvation and overwork as the communist regime emptied Cambodia's cities, exiling millions to vast collective farms in a bid to forge an agrarian utopia during its rule.
The Khmer Rouge also abolished money, religion and schools.
Duch's lawyers argued that years spent imprisoned without trial by another court — he was first arrested by the government in 1999 — violated the law and was grounds for his release.
"Duch has been detained for eight years without trial.
(Read content ‘Prosecutors demand KRouge prison chief remains jailed (AFP)’…)