Up to two million Cambodians died during the Khmer Rouge rule over Cambodia in the late 1970s, but none of the group’s leaders has ever been brought to trial.
Five former top officials are being held awaiting trial before the special tribunal, charged with war crimes and crimes against humanity.
The Khmer Rouge’s former supreme leader, Pol Pot, known as ‘Brother Number One’, died in his jungle hideout in 1998.
Tribunal officials expect the trial process to last at least until 2011.
However, critics say the advanced ages and frail health of all the accused means that, even if the trials do eventually go ahead, any sentences handed down are likely to be little more than symbolic.