Both Ieng Sary and his wife will be tried by the genocide court
Former Khmer Rouge foreign minister Ieng Sary has appeared before Cambodia's genocide tribunal to appeal against his detention.
The 82-year-old has been charged with war crimes and crimes against humanity committed during the Maoist regime's four-year rule in the late 1970s.
He is one of five former leaders of the Khmer Rouge being detained by the UN-backed tribunal.
Some 1.7 million people are thought to have died under the brutal regime.
Hundreds of thousands starved as the Khmer Rouge tried to create an agrarian society. Many others perceived as educated were tortured and executed.
Trials are expected to begin later in the year.
About 300 people attended the hearing at the court in the Cambodian capital, Phnom Penh.
WHO WERE THE KHMER ROUGE?
Maoist regime that ruled Cambodia from 1975-1979
Founded and led by Pol Pot, who died in 1998
Abolished religion, schools and currency in a bid to create agrarian utopia
Up to two million people thought to have died from starvation, overwork or execution
Ieng Sary is the most prominent surviving Khmer Rouge leader - and is still viewed as an influential and respected figure in parts of Cambodia, reports the BBC's Guy Delauney from Phnom Penh.
He received a royal pardon 12 years ago after reaching a deal with the government that resulted in the eventual surrender of the Khmer Rouge.
His lawyers say this is why he should not be facing charges now. They will also argue that a trial would amount to double jeopardy.
The Vietnamese-backed forces which ousted the Khmer Rouge in 1979 tried Ieng Sary in absentia and found him guilty of genocide. That verdict was overturned by the pardon.
But Cambodians who survived Khmer Rouge prison camps feel particularly strongly about the former foreign minister, our correspondent adds.
Many of them were well-educated people who returned to the country after personal appeals from Ieng Sary to help rebuild Cambodia.
They were arrested on arrival, and thrown into brutal detention centres.
Ieng Sary's wife, former social welfare minister Ieng Thirith, has also been charged by the genocide court.
The scene in the Phnom Penh courtroom