Screams from soldiers being tortured, overflowing cells, inhuman conditions, a regime of intimidation and murder. Inedible gruel, no communication with the outside world, and days marked off with a home-made calendar written on a box of tea.

This, according to a prisoner who was there, is what conditions are like inside Olenivka, the notorious detention centre outside Donetsk where dozens of Ukrainian soldiers burned to death in a horrific episode late last month while in Russian captivity.

Anna Vorosheva – a 45-year-old Ukrainian entrepreneur – gave a harrowing account to the Observer of her time inside the jail. She spent 100 days in Olenivka after being detained in mid-March at a checkpoint run by the pro-Russian Donetsk People’s Republic (DNR) in eastern Ukraine.

She had been trying to deliver humanitarian supplies to Mariupol, her home city, which the Russian army had besieged. The separatists arrested her and drove her in a packed police van to the prison, where she was held until early July on charges of “terrorism”.

Now recovering in France, Vorosheva said she had no doubt Russia “cynically and deliberately” murdered Ukrainian prisoners of war. “We are talking about absolute evil,” she said.

The fighters were blown up on 29 July in a mysterious and devastating explosion. Moscow claims Ukraine killed them with a US-made precision-guided Himars rocket. Satellite images and independent analysis, however, suggest they were obliterated by a powerful bomb detonated from inside the building.

Russia says 53 prisoners were killed and 75 injured. Ukraine has been unable to confirm these figures and has called for an investigation. The victims were members of the Azov battalion. Until their surrender in May, they had defended Mariupol’s Azovstal steel plant, holding out underground.

A day before the blast, they were transferred to a separate area in the camp’s industrial zone, some distance from the grimy two-storey concrete block where Vorosheva shared a cell with other women prisoners. Video shown on Russian state TV revealed charred bodies and twisted metal bunk beds.

“Russia didn’t want them to stay alive. I’m sure some of those ‘killed’ in the explosion were already corpses. It was a convenient way of accounting for the fact they had been tortured to death,” she said.

Male prisoners were regularly removed from their cells, beaten, then locked up again. “We heard their cries,” she said. “They played loud music to cover the screams. Torture happened all the time. Investigators would joke about it and ask inmates, ‘What happened to your face?’ The soldier would reply, ‘I fell over’, and they would laugh.

“It was a demonstration of power. The prisoners understood that anything could happen to them, that they might easily be killed. A small number of the Azov guys were captured before the mass surrender in May.”

Vorosheva said there was constant traffic around Olenivka, known as correctional colony No 120. A former Soviet agricultural school, it was converted in the 1980s into a prison, and later abandoned. The DNR began using it earlier this year to house enemy civilians.

Captives arrived and departed every day at the camp, 20km south-west of occupied Donetsk, Vorosheva told the Observer. Around 2,500 people were held there, with the figure sometimes rising to 3,500-4,000, she estimated. There was no running water or electricity.

The atmosphere changed when around 2,000 Azov fighters were bussed in on the morning of 17 May, she said. Russian flags were raised and the DNR colours taken down. Guards were initially wary of the new prisoners. Later they talked openly about how they were going to brutalise and humiliate them, she said.

“We were frequently called Nazis and terrorists. One of the women in my cell was an Azovstal medic. She was pregnant. I asked if I could give her my food ration. I was told, ‘No, she’s a killer’. The only question they ever asked me was, ‘Do you know any Azov soldiers?’”

Conditions for the female inmates were grim. She said they were not tortured but received barely any food – 50g of bread for dinner and sometimes porridge. “It was fit for pigs,” she said. She suspected the prison governor siphoned off money allocated for meals. The toilets overflowed and the women were given no sanitary products. The cells were so overcrowded they slept in shifts. “It was tough. People were crying, worried about their kids and families.” Asked if the guards ever showed sympathy, she said an anonymous person once left them a bottle of shampoo.

According to Vorosheva, the camp’s staff were brainwashed by Russian propaganda and considered Ukrainians to be Nazis. Some were local villagers. “They blamed us for the fact that their lives were terrible. It was like an alcoholic who says he drinks vodka because his wife is no good.

“The philosophy is: ‘Everything is horrible for us, so everything should be horrible for you’. It’s all very communist.”

Ukraine’s president, Volodymyr Zelenskiy, has called the explosion “a deliberate Russian war crime and a deliberate mass murder of Ukrainian prisoners of war”. Last week, his office and Ukraine’s defence ministry gave details of clues which they say point to the Kremlin’s guilt.

Citing satellite images and phone intercepts and intelligence, they said Russian mercenaries from the Wagner group carried out the killings in collaboration with Vladimir Putin’s FSB spy agency. They point to the fact a row of graves was dug in the colony a few days before the blast.

The operation was approved at the “highest level” in Moscow, they allege. “Russia is not a democracy. The dictator is personally responsible for everything, whether it’s MH17, Bucha or Olenivka,” one intelligence source said. “The question is: when will Putin acknowledge his atrocities.”

One version of events being examined by Kyiv is that the blast may have been the result of intra-service rivalries between Russia’s FSB and GRU military intelligence wings. The GRU negotiated Azovstal’s surrender with its Ukrainian army counterpart, sources suggest – a deal the FSB may have been keen to wreck.

The soldiers should have been protected by guarantees given by Russia to the UN and the International Committee of the Red Cross that the Azov detainees would be properly treated. Since the blast, the Russians have refused to give international representatives any access to the site.

Vorosheva said the Red Cross were allowed into the camp in May. She said the Russians took the visitors to a specially renovated room and did not allow them to talk independently to the prisoners. “It was a show,” she said. “We were asked to give our clothes’ size and told the Red Cross would hand out something. Nothing reached us.”

Other detainees confirmed Vorosheva’s version of events and said the Azov soldiers were treated worse than civilians. Dmitry Bodrov, a 32-year-old volunteer worker, told the Wall Street Journal the guards took anyone they suspected of misbehaviour to a special disciplinary section of the camp for beatings.

They emerged limping and moaning, he said. Some captives were forced to crawl back to their cells. Another prisoner, Stanislav Hlushkov, said an inmate who was regularly beaten was found dead in solitary confinement. Orderlies put a sheet over his head, loaded him into a mortuary van and told fellow inmates he had “committed suicide”.

Vorosheva was freed on 4 July. It was, she said, a “miracle”. “The guards read out the names of those who were going to be freed. Everyone listened in silence. My heart leaped when I heard my name. I packed my things but didn’t celebrate. There were cases where people were on the list, got out, then came back.”

She added: “The people who run the camp represent the worst aspects of the Soviet Union. They could only behave well if they thought nobody was looking.”