A Spanish judge has launched a criminal investigation into the fate of tens of thousands of people who vanished during the civil war and Franco dictatorship.
Judge Baltasar Garzon - Spain's top investigating judge - has also ordered several mass graves to be opened.
One is believed to contain the remains of the poet Federico Garcia Lorca, who was murdered by fascist forces at the start of the war in the 1930s.
Correspondents say the historic ruling will be controversial in Spain.
They say there has been a tacit agreement among political parties not to delve too deeply into the civil war and Franco era.
In his 68-page ruling, Judge Garzon says that Francoists carried out "illegal permanent detentions" which he says falls within the definition of crimes against humanity.
He refers to 114,000 people who disappeared during a 15-year period after the outbreak of war in 1936.
The BBC's Steve Kingstone, in Madrid, says that never before has Spain's civil war been investigated by a judge.
And in using the phrase "crime against humanity" Judge Garzon is taking a highly controversial step.
Judge Garzon told the BBC: "These days, crimes against humanity are a burning issue, wherever you look in the world, be it Afghanistan, Iraq or Darfur - enough countries to make you realise that this theme never ceases to make the news, just as the fight against this scar, this impunity never ceases.
"And if we are referring to the investigations being carried out in Spain in relation to universal justice or eras gone by, then justice needs to follow its course within the parameters of the law. That is what we judges try to do."
The civil war was triggered by the military uprising of General Francisco Franco, whose supporters are said to have systematically eliminated left-wing opponents, even after the war was won in 1939.
Judge Garzon's document names Gen Franco and 34 of his senior aides as the instigators of the alleged crimes.
He even asks that their death certificates be produced, to prove that they can no longer face prosecution.
The judge has also asked Spain's interior ministry to provide names of senior members of the fascist Falange Party, which supported Franco, with a view to possible prosecutions.
He has ordered the opening of 19 mass graves, believed to contain victims of the Franco regime. The remains of the poet Lorca are thought to be buried in the southern province of Granada.
Judge Garzon is famous for bringing crimes-against-humanity cases against figures such as former Chilean dictator Augusto Pinochet.
Last year he was asked by the families of some of those who vanished during the Spanish civil war or during Franco's dictatorship to help find the remains of their loved ones, and clarify the circumstances of their deaths.
An estimated 500,000 people died in the civil war.