Ethiopia’s new prime minister has been urged to investigate a raft of gruesome torture and abuse allegations involving senior officials in the country’s most notorious prison.

Jail Ogaden, officially known as Jijiga central prison, is home to thousands of prisoners and lies at the heart of Jigjiga, the capital of Ethiopia’s eastern Somali region.

According to a report by Human Rights Watch, published on Thursday, prisoners are routinely brutalised and denied access to adequate medical care, family, lawyers, and sometimes food. The report alleges that many have never been convicted of any crime.

Former prisoners claimed they saw people dying in their cells after being tortured by officials.

The report provides the most extensive catalogue to date of human rights abuses in eastern Ethiopia under Somali regional president Abdi Mohamed Omar, commonly known as Abdi Iley, who has governed Ethiopia’s second largest province since 2010.

A spokesman for the regional government dismissed the allegations as “baseless”.

Based on almost 100 interviews, including 70 former prisoners of Jail Ogaden, the study documents alleged abuses including rape, sleep deprivation, long-term arbitrary detention, collective punishment and forced confessions between 2011 and early 2018.

It highlights, in particular, the role of a 40,000-strong Somali special police unit known as the Liyu, which Abdi, then head of regional security, established in 2008 as part of a brutal counter-insurgency campaign targeting the Ogaden National Liberation Front, a secessionist rebel group. Most Jail Ogaden inmates are accused of some affiliation to the group, according to Human Rights Watch, although a regional government spokesman told the Guardian that all ONLF prisoners had now been released.

“Torture in detention is a serious problem throughout Ethiopia, but Jail Ogaden is in a class of its own,” said Felix Horne, the report’s author. “In my years of researching human rights abuses in Ethiopia, I haven’t seen such a non-stop cycle of torture, humiliation, and other abuses in detention as we found when researching Jail Ogaden.”

Ethiopia’s prime minister, Abiy Ahmed, took office in April following the unexpected resignation of his predecessor, Hailemariam Desalegn, in February. He has since embarked on a programme of reforms, including the release of thousands of prisoners and the pardoning of high-profile activists and politicians on death row.

Abiy’s government has closed the infamous Maekelawi detention centre in Addis Ababa, and in a June speech to parliament he took the unprecedented step of admitting security personnel complicity in torture. His cabinet later recommended that the Ogaden National Liberation Front, along with two other rebel groups, should be removed from the country’s terror list.

On Wednesday, just hours before the release of the HRW report, the government announced it had fired several senior officials from the federal prison service, including its director.

But Somali region, one of the most underdeveloped parts of Ethiopia, remains especially troubled. Last year, hundreds of people lost their lives and nearly a million were displaced in ethnic violence along the border with neighbouring Oromia region, with Oromo residents blaming the Liyu for carrying out attacks. Oromo security forces have also been accused of abuses. Liyu attacks reportedly continued this year, with scores of homes burned in May according to reports.

Abdi’s critics accuse him of one-man rule and hold him responsible for the Liyu’s atrocities. “The personal behaviour of Abdi Iley is a threat to the unity and security of the country,” said Abdiwasa Bade, a Ethio-Somali academic at Addis Ababa University, adding that the president has “absolute power” in the region.

In May, regional security forces allegedly hung a young woman as punishment for her relatives’ outspoken criticism of Abdi.

In February, Abdi announced the release of 1,500 prisoners, though local activists claim only a handful have so far been freed. In a televised speech on 15 June, he announced further releases and claimed only 1,600 people remain behind bars in the region.

The report claims Jail Ogaden is subject to “almost no meaningful scrutiny or oversight” and that no one has ever been held accountable for torture or abuse. It calls on the Ethiopian government to set up a commission to investigate the prison and its senior officials, including Abdi.

The report also calls for wholesale reform of the Liyu police, echoing a statement by Amnesty International in May that demanded the Ethiopian government “immediately withdraw and disband” the controversial paramilitary.

“The previous federal administration turned a blind eye to all abuse and torture carried out by the region’s security forces,” said Atnaf Berhane, a human rights activist and blogger who in 2014 spent three months in Maekelawi, where he says he was routinely tortured.

“The new administration must establish an independent committee to investigate all human rights violations and bring the main actors to justice.”

Somali regional government spokesman Idris Ismael Abdi said the report was “a smear campaign cloaked under human rights concern”.

“The Ethiopian Somali government fully realises that the democratic order being built in Somali region is not free from mistakes and errors,” he said. “[But] the central jail of Somali region is a centre of rehabilitation […] equipped with modern beds and all other basic requirements […] Prisoners get education, healthcare and all other essential things just like an ordinary person.”

He denied the Liyu police were responsible for administering prisons in the region.