The number two leader in Cambodia's former Khmer Rouge regime, accused of orchestrating Cambodia's “killing fields,” told a U.N.-backed tribunal that he acted in the interest of the country and to protect it from a potential invasion by Vietnam.

The communist movement's chief ideologue, Nuon Chea, has denied charges that he committed genocide and crimes against humanity, during the group's reign of terror in the 1970s that resulted in the deaths of up to two million people.

In comments made to the court, Chea said the charges against him were unfair, adding that the regime only wanted to repel an invasion by Vietnamese forces that were seeking to colonize the country.

Chea is charged, along with former head of state Khieu Samphan and Khmer Rouge Foreign Minister Ieng Sary, in connection with crimes committed during the regime's rule, which lasted from 1975 to 1979.

Co-prosecutor Andrew Cayley said Tuesday that the three aging and sickly former leaders were the orchestrators of Cambodia's bloody “killing fields.”

The long-awaited U.N.-backed tribunal entered its second day Tuesday, with opening statements in the capital of Phnom Penh.

Prosecutors are expected to seek the maximum penalty of life in prison for the defendants, though many fear they will not live long enough to be brought to justice.

Nearly 1,000 visitors listened to prosecutors describe the charges during the first day of the hearings on Monday. The crimes are considered one of the worst atrocities of the 20th century.

Prosecution statements will be followed by two days of defense response. Actual testimony is scheduled to begin in early December. The trial is expected to last two years.