The murderous Khmer Rouge regime's most senior surviving leader was detained on Wednesday, taken from his home in the Cambodian jungle by police and officials from a UN-backed genocide court.
An AFP correspondent saw Nuon Chea being driven from his home in northwest Cambodia and put on a helicopter.
He was expected to be flown to the capital Phnom Penh.
"Nuon Chea has been shown a warrant but I don't know what it is for or what crimes he has been charged with," a source close to him told AFP while also boarding the chopper.
The 82-year-old, known as
Brother Number Two
, was Khmer Rouge leader Pol Pot's most trusted lieutenant and allegedly a key architect of the regime's horrific execution policies.
Police earlier blocked the road leading to Nuon Chea's house as officials from the genocide tribunal swept in at dawn.
Ou Boran, a relative, told AFP that authorities were searching through the house looking for documents and correspondence.
Nuon Chea is the first of a small group of former top cadres living freely in Cambodia to be questioned by tribunal authorities, prompting many who lost relatives to wonder why they have been left alone for so many years.
His rank in the communist hierarchy and alleged decision-making role would make him the most significant defendant to be tried for crimes committed under the 1975-79 regime by the tribunal, which was established last year.
Up to two million people died of starvation, disease and overwork, or were executed under the Khmer Rouge. The regime abolished religion, schools and currency, exiling millions to vast collective farms in a bid to create an agrarian utopia.