World-renowned Spanish judge Baltasar Garzon on Tuesday defiantly rejected charges of abuse of power for opening a probe into Franco-era crimes.

Garzon, who earned global fame with an attempt to extradite Chilean dictator Augusto Pinochet from London, found himself in the Supreme Court dock accused of breaching an amnesty for such crimes.

"The amnesty law refers to crimes of a political nature, in no way can it be said that crimes against humanity of the kind that were alleged could have any political nature," the 56-year-old judge said.

"As such it was not even necessary to make a reference to the amnesty law," he said on the opening day of his testimony.

Victims' families who filed the case in 2006 had described disappearances, illegal detentions and killings, which amounted "in some cases to crimes against humanity, genocide," he said.

Old wounds

The judge is being prosecuted for ordering the investigation in 2008 into the disappearance of 114,000 people during Spain's 1936-39 civil war and General Francisco Franco's subsequent dictatorship.

He is charged with exceeding his powers on the grounds that the alleged crimes were covered by an amnesty agreed in 1977 as Spain moved towards democracy two years after Franco's death.

If convicted, Garzon would not go to prison but could be suspended from the legal profession for up to 20 years, putting an end to his career.

The case has opened heated debated in Spain over a dark period of its history, and invited criticism from rights groups.

Judging the judge

"Garzon showed today that his decision to take up the investigation of the crimes of the Franco era was fully supported by international law," said Human Rights Watch lawyer Reed Brody, who was in the court.

"But the spectacle of a judge as a criminal defendant, having to justify his investigation into torture, killings and 'disappearances,' was itself an affront to principles of human rights and judicial independence," he said in a statement.

Garzon was testifying shortly after the Supreme Court refused to dismiss the case, which was opened following a private complaint filed by two right-wing groups.

Both the prosecution and defence agreed that the judge had done nothing wrong and the charges should be shelved.

But their submissions "do not have sufficient material weight to justify the cancellation of the investigation," said the ruling agreed by a majority of the seven judges.

People's hero

Garzon has become a hero to many human rights activists and victims of the Franco period.

More than 100 protesters rallied outside the court in his defence, waving banners reading "Justice" and displaying black-and-white photographs of Franco's alleged victims.

"We are here because we are ashamed that a judge who tried to judge the crimes of Francoism should be the first to sit on the bench of the accused and that the Supreme Court should try to condemn it," said one activist at the rally, Pio Maceda.

"We are here to show our support for justice and to demand justice," he said. "In the judiciary there are still some elements of the old regime, this case is an embarrassment to Spain and the world."

Garzon came to international prominence in 1998 when he ordered the extradition of Pinochet from Britain to face charges of human rights abuses.

The judge has also pursued members of the former dictatorship in Argentina, indicted Osama bin Laden, probed abuses at the US prison for terrorism suspects at Guantanamo Bay in Cuba, and probed tax fraud accusations, later dropped, against former Italian prime minister Silvio Berlusconi.

Garzon was suspended from his duties at the National Court, Spain's top criminal court, in May 2010 and currently works as a consultant at the International Criminal Court in The Hague.