More than 1.7 million people died under the regime, from overwork, starvation or execution, in one of the worst atrocities of the 20th century.
In the countryside, especially in former Khmer Rouge areas, the verdict was met with mixed reactions.
The first phase of a two-phase trial began for Nuon Chea and Khieu Samphan in late 2011, in a trial that included more than 1,000 witnesses and 7,000 documents.
Nuon Chea, Khieu Samphan convicted of crimes against humanity three-and-a-half decades after communist group's bloody rule left nearly quarter of Cambodian population dead.
The tribunal sentenced Nuon Chea and Khieu Samphan, who are in poor health, to life in prison
Nuon Chea, 87, and Khieu Samphan, 82, are facing charges for their roles as leaders within the movement
Thursday’s verdict will be a landmark moment for the court, which has so far only tried one other Khmer Rouge member.
Aging leaders Nuon Chea and Khieu Samphan are facing atrocities crimes charges in two phases of a trial that was broken apart to expedite the process.
First phase of trial, which is coming to an end, has focused on forced exodus of Phnom Penh in 1975 - and now many are hopeful justice will be served.
Cambodia's former Khmer Rouge president, Khieu Samphan, arrived in court on Wednesday (July 30) for an initial hearing on charges for genocide, crimes against humanity, and grave breaches of the 1949 Geneva Conventions.
Ex-head of state Khieu Samphan, chief ideologue Nuon Chea face genocide charges at the trial that begins Wednesday in Phnom Penh.
Chum Mey sells books about the Khmer Rouge, and about himself, in what is now called the Tuol Sleng Genocide Museum.