Despite the sentence, Kaing Guek Eav, alias Duch, will serve no more than 19 years. The judge took off five years for the time Duch was illegally detained before the U.N.-backed tribunal was established, and another 11 years for the time he has already served behind bars.
The verdict against Duch also convicted him of crimes against humanity, murder and torture.
It was a historic first for the U.N.-backed war crimes tribunal. The tribunal began its work in 2007 after a decade of on-and-off negotiations between the United Nations and Cambodia over the structure and functioning of the court.
Duch, 67, was the head of the S-21 prison. Few people brought to the prison made it out alive; only about a dozen were found by the Vietnamese who invaded Cambodia in 1979.
"It's clear that he will never be a free man again," said Youk Chhang, director of Documentation Center of Cambodia.
Prosecutors had asked for a 45-year sentence with 5 years' credit for time served.
The judge, in announcing Duch's sentence, said he took into consideration that the defendant had expressed remorse, admitted responsibility and cooperated with the court. The judge also took into account the "coercive environment" of the Khmer Rouge, he said
Duch pleaded guilty, but said he was only following orders and asked for forgiveness.
In the last week of the trial, he argued that international law did not apply to him because he was following orders.
When the verdict was read, spectators in the packed courtroom turned to each other and discussed in hushed tones the significance of the sentence.
"Thirty-five years. You can't return the lives of the people that were lost. But it's a start," said Collin Sam, 22, a Cambodian-American from Long Beach, California, who lost all her family members on her father's side during the regime.
Outside the courtroom, hundreds of Cambodians gathered for the verdict and millions more watched as it was televised live.
"Today is a historic, important day for the people of Cambodia," said Reach Sambath, the spokesman for the court.
At least 1.7 million people -- nearly a quarter of Cambodia's population -- died under the 1975-1979 Khmer Rouge regime, from execution, disease, starvation and overwork, according to the Documentation Center of Cambodia.
Another four of the ultra-Maoist regime's former leaders are waiting to see if they will stand trial before a U.N.-backed tribunal for war crimes and crimes against humanity.
Chhang, director of the documentation center, said the verdict may not suit everyone, but he thinks Cambodians
will be able to turn over a new leaf.
"This is what we have, and then we must move [on]. We have our own identity now, our own family, our own society now. We have to build it, make it strong, to prevent [the past] from happening" again," he said.