Chief Khmer Rouge inquisitor Kang Kek Ieu was charged with crimes against humanity today.
Kang Kek Ieu, also known as Duch, is the first of Pol Pot's henchmen to be formally accused over the deaths of 1.7 million people in Cambodia's "Killing Fields".
After a morning hearing before a tribunal of domestic and international judges, the schoolteacher (65) and born-again Christian was taken into custody of the court, a statement said. It did not elaborate.
The long-awaited $56 million Cambodian-United Nations tribunal into atrocities committed during the Khmer Rouge's 1975-79 reign of terror has its own detention centre on the outskirts of Phnom Penh.
Chief Khmer Rouge inquisitor Kaing Khek Iev
Duch has confessed to committing multiple atrocities during this time as head of the capital's notorious Tuol Sleng, or S-21, interrogation centre.
The former prison chief is expected to be a key witness in the trial of other senior Khmer Rouge cadres and had been in a military prison since 1999.
At least 14,000 people deemed to be opponents of Pol Pot's "Year Zero" revolution passed through Tuol Sleng's gates. Fewer than 10 are thought to have survived.
Most victims were tortured and forced to confess to a variety of crimes, mainly being CIA spies, before being bludgeoned to death in a field on the outskirts of the city. Women, children and babies were among those butchered.
Earlier this month, prosecutors lodged formal cases against five suspects, who have not been named.
Besides Duch, they are widely thought to be "Brother Number Two" Nuon Chea, Khmer Rouge foreign minister Ieng Sary, former president Khieu Samphan and Meas Muth, a son-in-law of Pol Pot's military chief, Ta Mok, who died last year.
"Brother Number One" Pol Pot died in April 1998 in a Khmer Rouge redoubt in mountains along the Thai border.