A Spanish court heard testimony about Franco-era atrocities for the first time in history Tuesday -- but the man in the dock was a judge who investigated the crimes, not an alleged culprit.

The 56-year-old judge, Baltasar Garzon, who earned global fame with an attempt to extradite Chilean dictator Augusto Pinochet from London, is accused of abuse of power for looking into the dark period.

Garzon is charged with overreaching his powers by trying to prosecute the atrocities despite an amnesty. He could not be jailed but risks a 20-year ban from the legal profession that would effectively end his career.

The first witness, Maria Martin, recalled how in 1936 when she was just six her mother was jailed and then shot dead, and her body dumped into a mass grave on the side of a road in the central town of Pedro Bernardo.

"They released her to testify in court but then on the way they killed her, they killed 27 men and three women," the 81-year-old, who relies on a walker to get around, said in a broken voice.

Martin said her family has been fighting to recover the remains of her mother ever since.

"Until the day he died in 1977 my father wrote to the local authorities to try to recover the body. They told him: 'Go away, leave us in peace or we will do to you what we did to her'," she told the Supreme Court.

The defence has called 22 witnesses to testify for the families of victims, many of them buried in unmarked mass graves dotted across the country, some forgotten for decades.

"For the first time, these people will be able to tell a court what the dictatorship inflicted on them," said Emilio Silva, head of a group that tries to help people find the remains of the missing.

Pino Sosa Sosa, 75, told the court of how her father was taken away from from their home in the town of Aruca in the Canary Islands and never seen again.

"When they took my father I was very young. They took the bread and salt which we had at home because my mother was was sick and she was looking for him," she said.

Pino said her mother refused to sign the death certificate issued for her father "because she said they took him alive and she was searching for him alive".

Garzon 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.

For relatives of the victims "seeing how the judge who tried to help them is being criminalized is terrible," said Silva.

"But there is a positive part. That people from small towns tell the Supreme Court what happened to them, what was done, is impressive," he added.

Garzon opened the inquiry in response to a complaint filed by victims' families in 2006 which described disappearances, illegal detentions and killings during the Franco era.

The judge told the court Tuesday that 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.

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

He 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 looked into 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.