The Khmer Rouge's former head of state, Khieu Samphan, has been charged with war crimes and crimes against humanity at a UN-backed tribunal in Cambodia.
Khieu Samphan is the fifth Khmer Rouge official to be detained
The 76-year-old was earlier arrested at a hospital in the capital, Phnom Penh, and taken to face the panel of judges.
He is the fifth person to be targeted by the court, set up to bring surviving leaders of the Khmer Rouge to justice.
More than one million people are thought to have died between 1975 and 1979 under the brutal Maoist regime.
Khieu Samphan 's lawyers have said they will appeal against his detention, a tribunal spokesman said.
Khieu Samphan's arrest had been widely expected.
WHO WERE THE KHMER ROUGE?
Maoist regime that ruled Cambodia from 1975-1979
Founded and led by Pol Pot, (above) who died in 1998
Abolished religion, schools and currency in a bid to create agrarian utopia
Brutal regime that did not tolerate dissent
More than a million people thought to have died from starvation, overwork or execution
A former guerrilla fighter, he became the president of Democratic Kampuchea - as Cambodia was then known - after the Khmer Rouge came to power. He was a close confidant of leader Pol Pot.
He has long claimed that his position was ceremonial, and in a recently published book he denied responsibility for policies to starve people and orders to carry out mass killings.
Last week, amid reports that his detention was imminent, he was flown to hospital in Phnom Penh after apparently suffering a stroke.
Early on Monday, police entered the hospital and drove the former leader to the special courts to appear before a panel of investigating judges.
Khieu Samphan's detention completes the initial round-up of suspects by the tribunal, which was established last year after years of delay.
Prison chief Duch (or Kang Kek Ieu), charged in July with crimes against humanity
Khmer Rouge second-in-command Nuon Chea, charged in September with war crimes and crimes against humanity
Foreign Minister Ieng Sary, charged in November with war crimes and crimes against humanity
Social Affairs Minister Ieng Thirith, charged in November with crimes against humanity
Head of State Khieu Samphan, arrested in November, yet to be charged
Former Foreign Minister Ieng Sary and wife Ieng Thirith, the social affairs minister, were arrested last week and charged with crimes against humanity.
Pol Pot's second-in command, Nuon Chea, and Kang Kek Ieu - known as Duch - the head of the notorious Tuol Sleng prison, are also facing similar charges.
Their trials are expected to begin next year.
Under the Khmer Rouge, more than one million people died from starvation or overwork as leaders strove to create an agrarian utopia.
Hundreds of thousands of the educated middle-classes were tortured and executed in special centres.
Khmer Rouge founder Pol Pot died in 1998 and many fear that delays to the judicial process could mean that more of the regime's elderly leaders are never brought to justice.