Survivors of Cambodia's Khmer Rouge regime have held a remembrance ceremony a day before a United Nations-backed tribunal tries three accused architects of some of the 20th century's worst atrocities.

The survivors and relatives of the victims gathered Sunday at the Choeung Ek Genocide Center in the capital, Phnom Penh.

More than three decades after the end of the regime, the tribunal on Monday will begin hearing opening statements in the case against Khmer Rouge head of state Khieu Samphan, reputed chief ideologue Nuon Chea and Khmer Rouge Foreign Minister Ieng Sary.

The charges against the three include crimes against humanity, genocide and torture. The Khmer Rouge ruled from 1975 to 1979, when up to 2 million Cambodians, or one quarter of the population, are estimated to have died.

A fourth defendant, Ieng Thirith, was ruled unfit to stand trial last week because she has Alzheimer's disease. She is

Ieng Sary's wife and served as the regime's minister for social affairs.

The Khmer Rouge's supreme leader Pol Pot died in 1998 in a jungle camp, where he was held prisoner after his former

comrades turned on him.