Khmer Rouge 'Brother Number Two' Nuon Chea, Pol Pot's top surviving henchman, was detained today by police and security guards from the UN 'Killing Fields' tribunal and taken by helicopter to Phnom Penh.
He was then driven to the tribunal compound, witnesses said.
Investigating judge You Bunleng said the grey-haired octogenarian, who is known to be in poor health, was "being brought to be questioned" after a large squad of security men was sent to his wooden home in a forest near the Thai border.
"Special forces, including police and military, came to surround my father's house early this morning," his son, Nuon Say, said. At least three Westerners questioned the ageing guerrilla leader, he added.
"My dad seems to have no worries, but my mother is worried about him," Nuon Say said.
About 15 police officers, including a western security guard working for the joint Cambodian-United Nations Khmer Rouge tribunal, blocked access to the house, Noun Say said.
Nuon Chea is accused of being the surviving Khmer Rouge commander most responsible for the atrocities of the 'Killing Fields', in which an estimated 1.7 million people died.
The $56 million tribunal charged chief Khmer Rouge inquisitor Duch with crimes against humanity in July, the first formal indictment of any of the top leaders of the ultra-Maoist guerrillas who overran the capital in 1975.
Their "Year Zero" revolution was meant to transform the heavily forested Southeast Asian nation into an agrarian peasant utopia. Instead it descended into the nightmare of the "Killing Fields", one of the darkest chapters of the 20th century.
The Beijing-backed regime was toppled by invading Vietnamese troops in 1979 and Pol Pot died in the last Khmer Rouge redoubt of Anlong Veng in 1998.
Prosecutors have launched formal cases against four top leaders besides Duch, but have not named them.