Cambodians marked the annual Day of Anger to remember victims of the Khmer Rouge terror as the regime's top torturer was tried by a UN-backed genocide tribunal.

About 2,000 Cambodians, including hundreds of Buddhist monks, gathered at Choeung Ek, a former Khmer Rouge "killing field" dotted with mass graves about 15km south of Phnom Penh.

Forty students re-enacted the torture and executions inflicted by the regime under whose mid-1970s rule about 1.7 million people perished.

Performers wore black uniforms, the standard attire of the Maoist-inspired movement. Some acted as executioners, swinging bamboo sticks at the heads of victims whose arms were bound behind their backs.

The performance was staged just metres away from a memorial filled with victims' skulls and mass graves where thousands of the executed were buried.

Relatives of the victims expressed hope that some of the surviving Khmer Rouge leaders would finally be punished by the ongoing tribunal.

Now being tried is Kaing Guek Eav, alias Duch, who commanded the notorious S-21 prison in Phnom Penh from where as many as 16,000 men, women and children are believed to have been tortured before being sent to Choeung Ek for execution.

Duch is the first senior Khmer Rouge figure to face trial, and the only one to acknowledge responsibility for his actions.

Senior leaders Khieu Samphan, Nuon Chea, Ieng Sary and Ieng Sary's wife, who are all detained, are likely to be tried in the next year or two.