Police had earlier cordoned off the street outside Ieng Sary's home in the Cambodian capital, Phnom Penh, at about 5:30am.
Together with tribunal officials they spent about three hours inside the house before taking him away.
Ieng Sary's arrest had been widely anticipated as one of five unnamed suspects earlier listed by tribunal prosecutors.
Two of them, Nuon Chea, the former Khmer Rouge ideologist, and Kaing Khek Lev, better known as Duch, the former head of the notorious S-21 or Tuol Sleng prison, have already been taken into custody.
An estimated two million Cambodians died of hunger, disease, overwork and execution during the Khmer Rouge's rule between 1975 and 1979.
Like other surviving Khmer Rouge leaders, the 77-year-old Ieng Sary who served as deputy prime minister as well as foreign minister, has repeatedly denied responsibility for any crimes.
In Bangkok, Thailand, for a medical check-up in October, Ieng Sary told The Associated Press: "I have done nothing wrong. I am a gentle person.
"I believe in good deeds. I even made good deeds to save several people's lives. But let them [the tribunal] find what the truth is."
According to a July 18 filing by the prosecutors to the tribunal's judges, Ieng Sary, "promoted, instigated, facilitated, encouraged and/or condoned the perpetration of the crimes" when the Khmer Rouge held power.
|Ieng Sary has repeatedly denied |
committing any crimes [AP]
It said there was evidence of Ieng Sary's participation in planning, directing and co-ordinating the Khmer Rouge "policies of forcible transfer, forced labour and unlawful killings".
His 75-year-old wife participated in "planning, direction, co-ordination and ordering of widespread purges ... and unlawful killing or murder of staff members from within the ministry of social affairs", the prosecutors' filing said.
Critics of the UN tribunal say the process has been left too late and suspects may die before ever being brought before a court.
Pol Pot, the leader of the Khmer Rouge, died in 1998, while his military chief, Ta Mok, died in 2006.