A high-profile Spanish judge has gone on trial accused of violating a 1977 amnesty law by investigating civil war and Franco-era crimes.
The prosecution of Baltasar Garzon, who famously indicted late Chilean leader Augusto Pinochet, has been brought by two right-wing groups.
They say he overstepped his powers by investigating the disappearance of 114,000 people between 1936 and 1975.
Human rights groups have described the Supreme Court trial as a scandal.
Relatives of victims from the civil war and the subsequent dictatorship of General Francisco Franco were expected to gather outside the court in Madrid as the trial got under way.
The BBC's Tom Burridge in Spain says the case has wider implications for the country - and the idea that crimes in the past should not be the subject of investigations today.
This is one of three prosecutions, brought by private parties, facing Mr Garzon.
Last week he was in court on charges of illegally authorising police to bug the conversations of lawyers with clients.
He denied wrongdoing and said he had always sought to protect detainees' right to a fair defence.
His third trial, for which no date has been set, involves allegations that he took bribes over payments he allegedly received for bank-sponsored seminars in New York.
If convicted at any of the trials, Judge Garzon, 56, could be suspended from the legal profession for up to 20 years.
Judge Garzon gained a global reputation for his investigations into alleged human rights abuses committed around the world - most notably initiating the arrest in the UK of former Chilean military ruler Augusto Pinochet in 1998.
But to his critics, he is a left-wing busybody obsessed with self-promotion, correspondents say.
It was his promise in 2008 to investigate the disappearance of tens of thousands of people during the Franco era, including ordering the excavation of mass graves, that drew the most ire.
Clean Hands and Liberty and Identity, two organisations who have brought Tuesday's prosecution, say he should have heeded the amnesty agreed in 1977, two years after General Franco's death.
"Without doubt Judge Garzon has reopened wounds which we Spaniards - whatever our political beliefs - had totally recovered from," Miguel Bernard Ramon, of Clean Hands, told the BBC.
But many of the relatives of those who disappeared had pinned their hopes for justice on Judge Garzon.
Some 22 witnesses called by the defence to speak at the trial will testify for the families of victims.
"For the first time those people will be able to tell before a court what the dictatorship did to them," Emilio Silva, President of the Association for the Recovery of Historical Memory, told the AFP news agency.
"People from small villages will come to tell directly in the Supreme Court what happened, what was done to them. It is going to be extraordinary".
Reed Brody, a lawyer with the US-based group Human Rights Watch, said it was paradoxical that Judge Garzon should be put on trial for pursuing the crimes of dictatorship in his own country.
"Do the victims of Franco have less rights than the victims of Pinochet?" he said.