A former militia commander accused of overseeing murder, rape, enslavement and torture in Libyan detention centres has appeared at the international criminal court for a hearing that campaigners say is a landmark step towards “justice, truth, reparation and deterrence” of abuses of refugees trying to reach Europe from Africa.

The prosecution of Khaled Mohamed Ali El Hishri on charges of war crimes and crimes against humanity is the first to reach a courtroom resulting from the ICC’s investigation into crimes in Libya after the fall of Muammar Gaddafi in 2011.

Legal experts said the hearing on Tuesday, when judges will decide if there is sufficient evidence against Hishri for a trial, would be a “huge milestone”.

“It is a really important development,” said Allison West, a senior legal adviser at the European Center for Constitutional and Human Rights. “The is the first case in the [ICC’s] Libya investigation that has been ongoing for more than 15 years. It’s the first time we have got someone into custody.”

For survivors of abuse in Libya, the court hearing is a moment that survivors and victims “never thought would happen”, said David Yambio, who was held in Mitiga prison between 2019 and 2020 and accuses Hishri of beating him.

“Now [Hishri] is in front of the court, it sends a strong message to perpetrators wherever they are that they will be brought to account and justice will be delivered, even if it takes a long time,” Yambio said.

Hishri was arrested in Germany last year when, it is thought, he sought medical treatment for a family member. A senior officer in the Special Deterrence Force, a powerful armed group that ran detention sites in western Libya, the 47-year-old is accused of imposing a brutal regime at the Mitiga prison in Tripoli between February 2014 and at least mid-2020.

Such sites became infamous after Gaddafi’s fall as they filled with refugees detained in Libya or intercepted by the Libyan coastguard, which has been supported by the EU and member states since 2017, as they tried to reach Europe.

Amnesty International and other human rights groups have described “harrowing violations” that were “the horrifying consequences of Europe’s ongoing cooperation with Libya on migration and border control”. Human Rights Watch (HRW) said thousands of detainees were held in Mitiga in overcrowded, unhygienic cells and were systematically subjected to violent assaults.

At a hearing in December, prosecutors said there were reasonable grounds to believe Hishri personally killed one detainee, while a “significant number” of people died during his time at the prison, either from torture, being left outside in winter, untreated injuries or starvation.

Hishri is also accused of “personally torturing, mistreating, sexually abusing and killing detainees” and imposing “prison conditions aimed at increasing … suffering”. Detainees were variously shot, confined in small metal boxes and beaten with cables “sometimes for the entertainment and amusement of guards”, it is alleged.

Defence lawyers are expected to challenge the jurisdiction of the ICC and have called for Hishri’s release.

West said the case against Hishri would shine a new light on serious crimes against people in Libyan detention centres but that many alleged perpetrators remained at liberty. Eight ICC arrest warrants are still pending in connection with the violence in Libya that followed the fall of Gaddafi.

“One of the most significant things about this case, other than actually Hishri being in the dock, is that there are a lot of people who aren’t,” West said.

While some countries in Europe have investigated and prosecuted individuals for human smuggling and trafficking of people in Libya, these cases have not included charges of war crimes or crimes against humanity.

That Germany arrested Hishri was important, campaigners said. “One state finally … cooperated, in that they actually arrested and surrendered the suspect to the court, because there’s been many instances in the past where that hasn’t happened,” said Alice Autin, a HRW researcher.

One of Hishri’s alleged co-perpetrators at Mitiga prison was arrested in February last year in Italy but then released on a technicality and returned to Libya, prompting controversy in Italy and dismay at The Hague. “This case [with Hishri] is not against the most senior person of that prison but is … the first step in getting sort of to the perpetrators of that system,” Autin said.

Libya’s warring factions agreed a ceasefire in 2020 but the country remains divided between the administration of the military leader Khalifa Haftar in the east and the Tripoli-based government of national accord (GNA) led by the prime minister, Abdul Hamid Dbeibah, in the west.

The case of Hishri is politically sensitive. The Special Deterrence Force is allied to the internationally recognised GNA in Tripoli and nominally under the interior ministry.

The ICC, set up to be an independent international “court of last resort” for grave crimes that could not be dealt with locally, has been under immense pressure in recent years. The US has imposed sanctions on four judges for what it has called its “illegitimate actions” targeting the US and Israel, while the court’s chief prosecutor is being investigated over alleged sexual misconduct, which he denies.

Yambio called for an end to European programmes that support the Libyan militias. “The EU is complicit in these crimes,” he said.