(CNN) -- Hundreds of Cambodians packed a courtroom in Phnom Penh on Monday as three top Khmer Rouge leaders went on trial for their role during the bloody four-year regime in the mid-1970s.





The U.N.-backed Extraordinary Chambers in the Courts of Cambodia has scheduled four days of opening statements for the defendants, who are all in their 80s.





On trial are Ieng Sary, the former Khmer Rouge foreign minister; Khieu Samphon, the nominal head of state; and Nuon Chea, the prime minister, also known as Brother Number 2.





The head of the Khmer Rouge, Pol Pot, was known as Brother Number 1. He died in 1998, long before the U.N.-backed court came into existence.





A fourth defendant, Ieng Thirith, was ruled unfit to stand trial because she suffers from dementia and could be set free, prosecutor said. She is Sary's wife and served as the social affairs ministry during the regime.





Prosecutors have charged the defendants with crimes against humanity, grave breaches of the Geneva Conventions, genocide, homicide, torture and religious persecution.





Under Pol Pot's leadership, the Khmer Rouge regime was responsible for the deaths of millions of ordinary Cambodians during a four-year reign of terror that was eventually halted in 1979 by invading Vietnamese forces.





In 1975, the Khmer Rouge ordered people out of Phnom Penh, the capital, and other cities in Cambodia to work in the countryside.





It is said to be responsible for about 1.7 million deaths, roughly a quarter of the population at the time. Its stated aim was to create a Communist utopia, but instead the regime forced Cambodians into what has been described as a living hell.





Soldiers marched city-dwellers into the countryside and forced them to work as farm laborers. Those already living in rural Cambodia were expected to produce enough food for the country while teaching farming to those who had never done it before.





The regime abolished currency, and considered anyone with an education a threat. It did not allow modern medicine, and it isolated Cambodia to make it completely self-sufficient.





The results were disastrous: People died of starvation and disease as soldiers tortured and killed anyone suspected of being disloyal.





In the end, virtually everyone, including the soldiers, became a target due to the leadership's paranoia.
who they are accused of crimes against humanity? how many people died during the red regime? Which charges has been brought against defendants? How many defendants are  in their 80s? What are they charged with What number of people died during the rule What caused the fourth defendant to be unfit for trial? What was the fourth defendant ruled unfit to do
three top Khmer Rouge about 1.7 million Conventions, genocide, homicide, torture three Conventions, genocide, homicide, torture and religious persecution. 1.7 million dementia to stand trial
question: who they are accused of crimes against humanity?, answer: three top Khmer Rouge | question: how many people died during the red regime?, answer: about 1.7 million | question: Which charges has been brought against defendants?, answer: Conventions, genocide, homicide, torture | question: How many defendants are  in their 80s?, answer: three | question: What are they charged with, answer: Conventions, genocide, homicide, torture and religious persecution. | question: What number of people died during the rule, answer: 1.7 million | question: What caused the fourth defendant to be unfit for trial?, answer: dementia | question: What was the fourth defendant ruled unfit to do, answer: to stand trial