Nuon Chea, the Khmer Rouge commander known as "Brother No. 2," was detained by the tribunal courts Wednesday and charged with crimes against humanity and war crimes, officials said.
Nuon Chea was flown by helicopter from his home in Pailin and whisked from a military airport in Phnom Penh, by cars with tinted windows, to face charges at the tribunal courts.
His arrest marked the most significant action to date by the beleaguered Khmer Rouge tribunal.
Nuon Chea was taken past reporters into the court, where he was questioned, charged and placed in detention, according to a tribunal statement issued Wednesday evening.
Following Nuon Chea's extraction from Pailin, the former Khmer Rouge stronghold remained quiet.
Chheng Sopheak, an investigator for the rights group Licadho, said people were surprised, but not upset, by the arrest.
"We did not see people protest against the arrest. We have asked some people who say that actually it is a chance for Nuon Chea to clarify at the court to show justice," Chheng Sopheak said. "Some people are happy about the arrest."
Nuon Chea emerged as Pol Pot's top lieutenant following the victory of the Cambodian communists over US-backed forces in 1975, and scholars say he was instrumental in the policies that led to the deaths of 2 million people.
Nuon Chea has said he welcomes his day in court and told journalists in July he had no knowledge of the murderous purges undertaken by the regime when it controlled Cambodia, from 1975 to 1979.