The three most senior surviving commanders of the 1970s Khmer Rouge regime masterminded one of the 'worst horrors' of the 20th century, killing or enslaving millions of Cambodians, a U.N.-backed war crimes trial heard on Monday.

In opening statements in one of the world's most high-profile genocide trials, prosecutors said former President Khieu Samphan, ex-Foreign Minister Ieng Sary and second-in-command Nuon Chea had called the shots in the bloody 'Killing Fields' revolution that wiped out a quarter of the population and destroyed the lives of millions of survivors.

The three were top henchmen of Pol Pot, the late architect of the 'Year Zero' revolution under Democratic Kampuchea, as the regime was known. Between 1.7 and 2.2 million people died from 1975-79 of torture, execution, disease, overwork or starvation.

The defendants, who are charged with war crimes, crimes against humanity and genocide, showed no emotion as Cambodian co-prosecutor Chea Leang delivered powerful opening statements to a packed tribunal.

"Every Cambodian who was alive during this period was affected by the criminal system of oppression which these accused put in place. The death toll is staggering," Chea Leang said.

"The forced evacuations of Cambodian cities, the enslavement of millions of people in forced labor camps, the smashing of hundreds of thousands of lives in notorious security centers and the killing fields, and the extermination of minorities, the countless deaths from disease, abuse and starvation -- these crimes ordered and orchestrated by the accused were among the worst horrors inflicted on any nation in modern history."

A fourth defendant, former Social Affairs Minister Ieng Thirith, was declared mentally ill and unfit for trial on Thursday. She remains in detention pending an appeal.

Their case is the second to be brought before the Extraordinary Chambers in the Courts of Cambodia (ECCC) and could be the last, with rights groups furious over the court's unexplained rejection of a third high-profile case and allegations of misconduct, incompetence and political interference directed at the U.N. and Cambodian authorities.

SEARCH FOR JUSTICE, CLOSURE

With the defendants in their 80s and in ailing health, many Cambodians feel they will die before a verdict is reached in an extremely complex case that could last years. Prosecutors are expected to ask for the maximum sentence of life imprisonment.

The Khmer Rouge era is still shrouded in mystery and most Cambodians just want to hear the truth about one of the world's most murderous and enigmatic regimes.

"We want justice so that the dead can finally close their eyes," Chum Noeu, 62, who lost 13 relatives under Khmer Rouge rule. "What is the truth behind all of torture and killings? What happened?"

The court has handed down just one sentence so far, a 35-year jail term, commuted to 19 years, for former prison chief Kaing Guek Eav, alias 'Duch,' over the deaths of more than 14,000 people at the torture center he ran. His appeal is set for February 3 next year.

The defendants have so far shown no willingness to cooperate with the tribunal and are expected to deny responsibility. Ieng Sary has taken a vow of silence.

Nuon Chea's lawyers sought to delay the trial on Monday by asking for trial chamber judge Silvia Cartwright's resignation, claiming she had held unofficial meetings with co-prosecutors, but Judge Nil Nonn rejected their move.

Nuon Chea wore dark sunglasses during the proceedings and is unlikely to confess, despite recorded comments he made in a documentary film "Enemies of the People," during which he said threats to the party line were 'destroyed' if they could not be 'corrected or re-educated'.

With a raft of resignations, long delays and expenditure expected to surpass $150 million by the end of the year, Cambodians have lost confidence in a tribunal that took more than a decade to establish.

Many just want to hear the defendants tell them why so many people had to die.

"I have been wondering for so long why this regime was so brutal, why they killed so many people," said Kim So, another survivor who lost 13 relatives, including his parents.

"I am wondering how human beings like Pol Pot and Ieng Sary could do this to other human beings."