Media playback is unsupported on your device Media caption The UN-backed trial of the three most senior surviving leaders of the Khmer Rouge, charged with crimes against humanity, has begun

The three most senior surviving leaders of Cambodia's genocidal Khmer Rouge regime have gone on trial.

They include Nuon Chea, also known as Brother Number Two. He was the right-hand man of the Maoist regime's supreme leader Pol Pot, who died in 1998.

The former leaders, now all in their eighties, face charges including genocide and crimes against humanity.

The Khmer Rouge regime fell in 1979, and the process of trying its senior figures has taken many years.

Cambodia originally asked the United Nations and the international community to help set up a tribunal into the genocide in the mid-1990s.

A joint tribunal was finally established in 2006 following long drawn-out negotiations between the Phnom Penh government and the UN - but to date only one person has been convicted.

'Absolute suffering'

The tribunal began hearing four days of opening statements on Monday. Court spokesman Lars Olsen described the day as a "major milestone", saying: ""Many people never thought it would happen."

At the scene Arriving at court this morning were the frail and the elderly - survivors of the Khmer Rouge, as well as former guards and regime officials. Some have come to give evidence. Others to simply observe the last chapter of the nightmare which claimed the lives of nearly two million people. The UN's British chief prosecutor, Andrew Cayley, said the proceedings would send a strong message around the world: "If you lead a country into this kind of disaster the past will catch up with you." But the trial - a joint enterprise between the UN and Cambodia - has been heavily criticised. Theary Seng, whose parents were killed by the Khmer Rouge, said putting three people on trial for the deaths of 1.7 million simply wasn't enough. Hell on Earth, documented in court

As well as Nuon Chea, the regime's former head-of-state Khieu Samphan and Ieng Sary, who was foreign minister and the international face of the organisation, are also on trial.

All three deny the charges they face.

Ieng Thirith, the former social affairs minister, had been set to go on trial with them but has been declared unfit due to health issues.

Prosecutors told the tribunal that the Cambodian people were in a "pitiful state" and their suffering "was absolute" during the regime's rule.

The Khmer Rouge "turned Cambodia into a massive slave camp, reducing an entire nation into prisoners living under a system of brutality that defies belief to the present day", said co-prosecutor Chea Leang.

The regime attempted to create an ideal communist society by forcing city residents to work as peasants in the countryside, and by purging intellectuals, middle class people and any supposed enemies of the state.

About 1.7 million people - about one-third of the population - are believed to have been murdered, or died of over-work, starvation or torture from 1975 to 1979.

Hundreds of people - including monks, students, regime survivors and former cadres - packed the court's public gallery for the first of four days of opening statements in the landmark case.

"I feel very happy. I came here because I want to know the story and how it could have happened," 75-year-old farmer Sao Kuon, who lost 11 relatives under the Khmer Rouge, told the AFP news agency.

Who were the Khmer Rouge? Maoist regime that ruled Cambodia from 1975-1979

Led by Saloth Sar, better known as Pol Pot

Abolished religion, schools and currency in effort to create agrarian utopia

Up to two million people thought to have died of starvation, overwork or by execution

Defeated in Vietnamese invasion in 1979

Pol Pot fled and remained free until 1997 - he died a year later

The process has been broken up into several mini-trials, with the first hearing set to judge on the offence of enforced removal of people from the cities.

The BBC's Guy De Launey in Phnom Penh says the defendants are old and frail, and concern that they might die has forced the tribunal to split the cases in the hope of gaining at least one conviction.

But it is unclear how much the court will hear from the three accused.

Ieng Sary has already said he does not intend to testify, and Nuon Chea walked out of an earlier hearing.

Pol Pot died in 1998 before facing a full trial for his crimes.

The only senior Khmer Rouge figure to be convicted so far is Kaing Guek Eav - better known as Comrade Duch.

He was head of the notorious Tuol Sleng prison - a torture facility located in a school building - where he presided over the torture and murder of thousands of people.