(CNN) -- A trial over one of Latin America's darkest moments is under way in Argentina, where 25 defendants are accused of crimes against humanity for their alleged role in a 1970s campaign of repression and violence.
Operation Condor was an organized campaign by the continent's right-wing governments to suppress and crush political opposition. On Tuesday, 25 former officials tied to the operation appeared in court for the first day of what officials say could be a trial of more than two years.
The clandestine nature of Operation Condor means that its full extent may never be known, but researchers estimate that 50,000 were killed, 30,000 were "disappeared" and presumed killed, and 400,000 were jailed, according to The Center for Justice and Accountability.
Chile under dictator Augusto Pinochet was central to the operation, which also included participation from Argentina, Uruguay, Brazil and Paraguay, the center says.
Tuesday's trial is the first time for such a large number of defendants in the case to be tried together. The defendants are Argentinean except for one Uruguayan, former Maj. Juan Cordeiro Piacentini.
Among the specific charges, 12 of the defendants, including former Argentine dictator Jorge Rafael Videla, are accused of 108 counts of human rights violations in several South American countries.
Videla is already serving a life sentence for human rights abuses during his tenure.
Another former Argentinian dictator, Gen. Reynaldo Benito Bignone, is among another group of defendants being charged with 35 specific cases of abuse under Operation Condor.
Bignone also was previously convicted of human rights abuses in another trial and is serving a 25-year sentence.
The trial is expected to last a minimum of two years and about 500 witnesses are expected to testify, the government's Center for Judicial Information said.