The BBC's Caroline Gluck in Phnom Penh: "The men's treatment has enraged many"

Khieu Samphan and Nuon Chea switched to the government side after striking a deal with Prime Minister Hun Sen.

The pair were principal architects of the late Pol Pot's revolution in Cambodia between 1975 and 1979 which cost an estimated 1.7 million lives.

Khieu Samphan, the former Khmer Rouge nominal leader

They have spent the past week receiving VIP treatment and visiting tourist areas of Cambodia as a guest of the government, but their special treatment has prompted criticism from Cambodians and human rights groups.

A spokesman said the government still wanted a trial but the two were free to go because no warrant existed for their arrest.

"Everyone is presumed innocent until they are proven guilty," government spokesman Khieu Kanharith is reported as saying.

"We say they are free to go until they are summoned by a court. If the court does summon them and they don't return, then they can be in contempt of court."

There is concern that once the leaders return to Pailin - an area controlled by another former Khmer Rouge leader, Ieng Sary - it could be difficult to bring them to a tribunal.

Hun Sen faces criticism by human rights activists

A leading human rights worker, Lao Mong Hay, of the Khmer Institute of Democracy, said the government should have detained the men while the UN decided whether to try them.

He believed the reason the government had not been willing to take legal action was pressure from countries like Vietnam, China and Thailand, all of which have been closely involved with the Khmer Rouge in the past.

A coalition of 17 human rights groups, the Human Rights Action Committee, is trying to get 500,000 signatures for a petition calling on the government to fully support the trial of the men for crimes against humanity.

The group says that no national reconciliation can ever take place until the movement's top leaders are brought to justice.

But correspondents say the burden of bringing the two leaders to justice now lies with the international community.

Experts hired by the UN have been looking into the possibility of setting up a tribunal to try Khmer Rouge leaders and are due to report at the end of this month.