Khieu Samphan and Nuon Chea were flown by helicopter from a former rebel stronghold to the capital Phnom Penh to surrender to the Prime Minister, Hun Sen, after he pledged they would not face trial for crimes against humanity.
They were asked by journalists, many of them Cambodians who lost family members, if they felt remorse for causing the deaths of up to 2 million people.
"Yes, sorry, very sorry," Khieu Samphan said. "We would like to apologise and ask our compatriots to forget the past so our nation can concentrate on the future," he said. "Let bygones be bygones."
After the Khmer Rouge revolutionaries emerged victorious in a civil war in 1975 they forced the population into slave labour camps. One Cambodian in five perished from overwork, starvation, disease and executions.
The Khmer Rouge and their chief, Pol Pot, were overthrown in 1979 by the Vietnamese, but spent the next two decades fighting on in the countryside.
Survivors of the "killing fields" yesterday dismissed the apology. Bun Sray, 38, a civil servant, said: "Millions of lives, including 20 of my relatives, were lost under their regime. Now I want to kill their wives and children and then say sorry."
Keo Rotana, 45, said: "More than 10 of my family were killed in their regime and Khieu Samphan says sorry." (AP)