A key witness has testified in the Italian trial of a former Argentinian army officer accused of murders and forced disappearances during Argentina’s 1976-83 military dictatorship.

Alberto Rivas, 69, flew from Buenos Aires to Rome to give evidence about dozens of kidnappings and incidents of torture and at least two murders allegedly ordered by the former Lt Col Carlos Luis Malatto, who is wanted by the Argentinian authorities and stands accused of five counts of crimes against humanity.

Since he fled Argentina in 2011, Malatto has been living as a free man in Italy. In 2014 Italy refused Argentina’s request to extradite him, but the following year the justice ministry approved his trial in Rome for the murders of Juan Carlos Cámpora, the rector of the University of San Juan; Angel José Alberto Carvajal, a communist party official; Jorge Bonil, a soldier; and Marie Anne Erize, a French-Argentinian model. Malatto maintains his innocence.

Erize was an activist and adherent to the Peronist movement, Monteneros, which opposed the military dictatorship of Jorge Videla. She was kidnapped on 15 October 1976 by three men in front of a bicycle shop and there has been no trace of her since.

More than 40 years later, Rivas has decided to reveal the alleged details of that kidnapping. “My bicycle was broken that day, but as soon as I entered the workshop, some men ordered me to lie down on the ground,” Rivas told the Guardian. “They had just blocked a woman, a French girl, and were dragging her outside. There was a blond man, bald, who was with them. He was the leader of these men and they called him Lieutenant Malatto.”

About two weeks later, on 30 October 1976, Rivas was arrested for theft and taken to the Chimbas prison, in the province of San Juan, where political prisoners of the Videla military regime were locked up. “They put me to work in the garden, near the entrance to the prison,” said Rivas, who landed in Palermo on Friday to hold a press conference about the case. “General Jorge Olivera – sentenced to life imprisonment for crimes against humanity – used to come in and out of the prison. Olivera was always with Malatto and bragged about raping the French model.”

Rivas said he also witnessed the murder of Angel José Alberto Carvajal. Carvajal had been kidnapped on 29 July 1977 and taken to prison. He died in Chimbas on 18 August 1977. The authorities said he killed himself, but murder has always been suspected.

“I saw the lifeless body of Carvajal being dragged by two soldiers,” said Rivas. “They raised the corpse to his knees and attached a rope to his neck. Then they made everyone believe he had committed suicide. But they killed him.”

After the 1976 coup, Argentina’s military set about systematically crushing any potential opposition and eventually murdered 30,000 people, almost all of them unarmed non-combatants. Pregnant prisoners were kept alive until they gave birth. Five hundred babies are believed to have been given to childless military couples to raise as their own.

Among the tasks assigned to Rivas in Chimbas prison was cleaning the torture room. “I’ll never forget the walls and floor of that cell, stained with the blood of hundreds of tortured innocents,” Rivas said.

He said he had decided to testify after 40 years because he had “realised later how important the truth was for the family of the desaparecidos [disappeared] seeking justice for their mothers, children, fathers and brothers.”

Rivas lived for two years under police escort. Eight shots were fired at his window in 2014 after he decided to collaborate with Argentinian prosecutors.

Last June, reporters from the Italian newspaper la Repubblica discovered Malatto in a tourist resort enjoying holidays in the province of Messina, even though he was on trial in Rome.

‘’We asked the Italian judiciary to monitor Malatto’s movements,” said Jorge Ithurburu, a lawyer for 24 Marzo, a Rome-based NGO. “We fear that he may escape justice.”

Eva Lerouc, 45, whose parents were kidnapped in 1976 by the regime, also testified before prosecutors on Thursday. Her father, Armando Lerouc, was killed and her mother, Elida Saroff, disappeared after the kidnapping. “This is unfair,” she said of the trial. “We’ll fight because Malatto has to be extradited. We seek justice.”

In 1985, two years after Argentina returned to democracy, the coup’s leader Videla was convicted of human rights abuses and crimes against humanity. Since then, more than 1,000 other former officers have been sentenced for the torture and extrajudicial murder of activists and political opponents and their families.

Many former officers continue to live in hiding in Europe, especially in Italy where many Argentinians have family ties. Malatto’s family originally hailed from Abruzzo and he holds an Italian passport.

Last July an Italian court sentenced 24 people to life in prison for their involvement in Operation Condor, in which the dictatorships of six South American countries conspired to kidnap and assassinate political opponents in each other’s territories.