Five former intelligence and military officials in Argentina have gone on trial on charges of murdering 65 people.
They are accused of kidnapping, torturing, and killing left-wing activists under the country's military rule between 1976 and 1983.
Human rights groups hope the trial will shed light on Operation Condor, a joint effort among South American military rulers aimed at suppressing opposition.
The five have denied the charges.
They include two former intelligence officers, Honorio Martinez Ruiz and Eduardo Ruffo, former Gen Eduardo Cabanillas, former Col Ruben Visuara, and former military intelligence agent Raul Guglielminetti.
A sixth man, former Vice Cmdr Nestor Guillamondegui, was excused from the trial on health grounds, court officials said.
They said his health would be monitored to determine if he could face trial at a later date.
The men are accused of having run a notorious detention centre in Buenos Aires.
More than 200 people are believed to have been kidnapped and taken to the secret prison, known as Automotores Orletti.
Most of the detained were from Uruguay, but survivors say it also housed prisoners from Chile, Paraguay, Bolivia and Cuba.
Condor was devised in 1975 by military officials from Argentina, Brazil, Bolivia, Chile, Paraguay and Uruguay.
Its aim was to silence the opposition by sending teams into other countries to track, monitor and kill dissidents.
A prosecutor said what happened at Automotores Orletti was "calculated and planned and amounted to a death sentence" for the prisoners.
Marcelo Gelman, the son of Argentine poet Juan Gelman, was one of those detained in the clandestine prison in 1976. His body was later found in a cement-filled drum dumped in a river.
His wife, Maria Claudia Garcia, was pregnant when she was abducted. She was taken to Uruguay, where she was disappeared.
Their daughter, Macarena Gelman, was raised by a police officer in Uruguay. She says she will testify at the trial, which is expected to last months.