Dec 29, 2006
Rodolfo Almiron was detained near Valencia on a warrant to face murder charges in Argentina.
He is a suspected member of Triple A, the anti-communist alliance that operated under the governments of Juan Peron and then his widow Isabel.
The group is blamed for the killings of 1,500 perceived government opponents.
The BBC's Daniel Schweimler says any mention of the Triple A still strikes terror into the hearts of many Argentinians today.
The group is held responsible for killing at least 1,500 perceived left-wing opponents of the government of Juan Peron and when he died in 1974, that of his widow Isabel who was toppled by Jorge Videla in a 1976 coup.
Rodolfo Almiron, said to be one of the leaders of Triple A, is alleged to have carried out many of the killings personally.
He fled to Spain in 1975 in the midst of chaos in Argentina, with left-wing factions battling against the right, the police and the armed forces.
The military took power shortly afterwards, ostensibly to restore order but imposing their own form of terror, killing at least 30,000 people over the next seven years.
Rodolfo Almiron, 71, thought he had escaped his past, living the last 30 years in comfort in Spain, our correspondent says.
However, last week a judge in Argentina ruled that the crimes which he has been accused of do not fall under any statute of limitations and therefore he could be tried.
Mr Almiron is expected to be transferred to Madrid's National Court in the next few days to start extradition proceedings, police said.
The arrest came as Argentina asked Spain to extradite a key figure in the military government.
Gen Ricardo Miguel Cavallo has been held in Madrid on charges of crimes against humanity for the past three years, but the high court decided last week it had no jurisdiction over him