The Spanish public prosecutor's office says it will seek a prison term of up to 17,000 years for former Argentine junta officer Ricardo Cavallo.
Ricardo Cavallo: In custody in Madrid since June 2003
The ex-naval captain is facing genocide and terrorism charges, which he denies.
Mr Cavallo was extradited to Spain from Mexico in June 2003 for alleged human rights abuses committed under Argentina's former military government.
Up to 30,000 people were killed or disappeared in Argentina during the "dirty war" of 1976-1983.
Many of the detainees passed through the notorious ESMA naval school - a secret torture centre.
In April last year, another former military officer from Argentina, Adolfo Scilingo, was sentenced in Spain to 640 years in prison for crimes against humanity.
Despite such long prison sentences being passed down in Spain, 30 years is the maximum that can be served under Spanish law.
During the Argentine military's campaign against left-wing insurgents - the "dirty war" - many of those arrested were tortured, drugged and thrown from aircraft into rivers and the sea.