Opposition parties and NGOs are demanding explanations from the Italian government after Rome released the head of Libya's judicial police, who had been arrested on an International Criminal Court (ICC) warrant over allegations of war crimes and crimes against humanity.

Osama Najim - also known as Almasri - is also the director of an infamous detention centre in Mitiga, near Tripoli, which Amnesty International says is the scene of "horrific violations committed with total impunity".

Mr Najim was freed due to a legal technicality, Italian media said, quoting interior ministry sources.

On Wednesday evening, the ICC said Mr Najim had been released by Italy "without prior notice or consultation with the Court" and issued another arrest warrant for him.

He had been arrested on Saturday in Turin, where he had attended a Juventus-Milan football match.

Mr Najim was flown back to Libya on Tuesday night on an Italian government flight. Images shared by local media showed a jubilant crowd welcoming him back with cheers and fireworks.

Shortly after Mr Najim was arrested internal ICC sources were already concerned Italy may release him, said Avvenire, the daily newspaper that first broke the story.

Italian opposition parties have expressed outrage at the release, which Rome has not yet commented on.

Matteo Renzi, former prime minister and leader of a centrist opposition party, asked justice minister Carlo Nordio "to account for his decision before MPs", while Arturo Scotto of the Democratic Party (PD) said Mr Najim was a "Libyan mafia boss" and that Italy - an ICC signatory - had to adhere to international treaties.

Libyan writer and journalist Khalil Elhassi said Italy should explain to the Libyan people why it had released "an extremely dangerous criminal" who he said was "known for taking part in torture of Libyans and migrants".

Two NGOs who work with migrants - Mediterranea Saving Humans and Refugees in Libya - said they had been left "stunned" by news of Mr Najim's release.

"Some of us have suffered torture in the Mitiga camp," the NGOs said in a statement, adding that "any crime committed on [detainees] will fall on the conscience of those who protected and freed a criminal like [Mr Najim]."

According to Amnesty International, Mr Najim headed the Judicial Police wing of the Mitiga prison since 2016 and, from 2021, oversaw various other prisons as director of the Reform and Rehabilitation Institution of the Libyan Judicial Police.

Libya has been splintered since the overthrow of Muammar Gaddafi in 2011, is currently divided between two rival governments, as well as by militias.

Right-wing Prime Minister Giorgia Meloni has made frequent trips to Libya and signed agreements with Tripoli.

In October, she said the two countries were bound "deep cooperation" and were working together to fight illegal immigration.

But Italian governments of all stripes have worked with Libyan authorities and militias for years, providing them with financial and technical support under controversial agreements to tackle illegal immigration, including training and funding for the Libyan coast guard which intercepts migrant boats.

As part of this agreement, Libya is expected to help reduce the number of migrants who pass through the country and arrive in Italy.

The people who are intercepted by the coastguard and brought back to shore are often imprisoned in detention camps, where they are subject to inhuman treatment and dire conditions.

Earlier this month, Italy released an Iranian engineer, Mohammad Abedini, who had been detained on suspicion of supplying drone technology that led to the deaths of US soldiers.

It is thought Abedini's release was linked to the case of Italian journalist Cecilia Sala, who had been freed from a Teheran jail days before.