Spain's Supreme Court cleared crusading human rights investigator Baltasar Garzon on Monday of abusing his authority when he decided to investigate the murders of more than 100,000 people by the forces of former dictator Francisco Franco.

Garzon, internationally known for ordering the arrest of former Chilean dictator Augusto Pinochet in 1998, had been charged with violating an amnesty for political crimes passed in 1977 to ease the transition to democracy after Franco's rule.

In its ruling, backed by six of the seven judges considering the case, the court said Garzon's legal action contained arguments we consider erroneous, but nothing that meant he was guilty of over-stepping the amnesty.

This erroneous application of law does not constitute the offence of exceeding authority, the ruling said.

The case rekindled a debate in Spain and beyond about whether more should be done to explore the darker events of the Franco regime which lasted from the end of the Spanish Civil War in 1939 until Franco's death in 1975.

Garzon said he had opened the investigation in 2008 at the request of victims' families and that international law backed him, as it had in the Pinochet case.

Human Rights Watch, one of a number of groups which backed Garzon's investigation, welcomed the acquittal but regretted the judge's expulsion from the bench in a separate case earlier this month.

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The Supreme Court has spared itself further embarrassment by rejecting these ill-advised charges, said Human Rights Watch lawyer Reed Brody.

The court disbarred Garzon for 11 years on February 9 for illegally recording defence lawyers' conversations with clients, a ruling that could effectively end the 56-year-old's career. He plans to appeal.

The U.N. human rights office said earlier this month Spain must probe crimes against humanity committed during the Franco era and must repeal its amnesty for perpetrators as there is no statute of limitations for such crimes.

The campaign to overturn Spain's amnesty law will now begin in earnest, Brody said.

A spokesman for victims' families welcomed the ruling, but said they still needed the backing of the courts - which Garzon tried to provide in 2008 - to search for the many unmarked graves in Spain which date from the Civil War.

After initiating his legal action, Garzon, a High Court judge, passed jurisdiction down to regional courts to help victims' families search for their loved ones.

Those cases remain in legal limbo due to uncertainty about which courts, if any, have the right to look into alleged Franco-era crimes. Monday's ruling did nothing to clarify that key point.

Dozens of courts in Spain received cases, some have initiated proceedings, others have done nothing and some have returned them to the High Court considering it to have jurisdiction, said Emilio Silva, who chairs the Association for the Recovery of Historical Memory.

The Supreme Court should with the greatest urgency ... state which court is competent so that the mass graves can be investigated, she added.

One of many victims whose whereabouts remain unknown is Federico Garcia Lorca, Spain's most famous 20th-century poet.

Legal group the International Commission of Jurists said Garzon's acquittal was a bitter victory since the holding of the trial amounted to a an attack on judicial independence.

Spain has to decide whether or not, at this point in history, it is ready to face its past and to allow victims, family members and Spanish society as a whole to know the truth about the human rights violations ... and to obtain proper redress for those wrongs, said the group's Belisario dos Santos Junior, who observed the trial.

The advocacy group Clean Hands which brought charges against Garzon for the Franco case in a private suit without the support of Spain's public prosecutor, said it would appeal the acquittal.

(Additional reporting by Teresa Larraz and Anna Valderrama; Writing by Martin Roberts and Sarah Morris; Editing by Robin Pomeroy)