'You can't imagine the conditions' - Accounts emerge of Russian detention camps

"You can't imagine how horrible the conditions were."

Oleksandr and Olena are two of the lucky few who recently managed to escape from Mariupol, which is now almost under full Russian control after weeks of bombardment.

The city is effectively sealed off from the world, and information about what is happening inside is difficult to confirm independently.

But the pair, and others, have given chilling accounts of life in Russia's so-called filtration camps, set up outside Mariupol to house civilians before they are evacuated.

Oleksandr and Olena, speaking from the relatively safe western city of Lviv, say they ended up at one of the centres when they tried to escape the city. After walking from their home to an evacuation point, they were driven to a Russian refugee hub at a former school in the village of Nikolske, north-west of Mariupol.

"It was like a true concentration camp," Oleksandr, 49, says.

The centres have been compared by Ukrainian officials to those used during Russia's war in Chechnya, when thousands of Chechens were brutally interrogated and many disappeared.

Oleksandr and Olena were fingerprinted, photographed from all sides, and interrogated for several hours by Russian security officers - "like in a prison", he says.

They worried that the Russians would look at their phones, and so they cleared all evidence from their devices of anything to do with Ukraine - including photos of their daughter in front of a Ukrainian flag.

They were right to worry. Oleksandr says that during their interrogation, Russian security officers examined photographs, phone call history and contact numbers on their devices for links with journalists or government and military officials.

"If a person was suspected of being a 'Ukrainian Nazi', they took them to Donetsk for further investigation or murder," says Oleksandr, although the BBC has not been able to verify this claim. "It was very dangerous and risky. Any small doubt, any small resistance - and they could take you to the basements for interrogation and torture. Everybody was afraid to be taken to Donetsk."

President Vladimir Putin has stated one of the aims of his invasion is to clear Ukraine of Nazis, and Russian propaganda has made numerous baseless allegations that Ukraine is somehow aligned with Nazism.

Eventually the pair were detained and put on a list for evacuation. But the ordeal did not stop there.

Elderly people slept in corridors without mattresses or blankets, Olena says. There was only one toilet and one sink for thousands of people. Dysentery soon began to spread. "There was no way to wash or clean," she says. "It smelt extremely awful."

Soap and disinfectant ran out on the second day they were there. Soon, too, did toilet paper and sanitary pads.

Olena and Oleksandr were told they had permission to leave on the 148th evacuation bus. But a week later, just 20 buses had left the facility.

In contrast, there were many buses organised to go to Russian territory. Authorities even tried to force the couple on to a coach heading east, they say. In the end Olena and Oleksandr felt compelled to seek the help of private drivers, who they feared could be Russians or collaborators.

"We didn't have any choice - either be forcibly deported to Russia or risk it with these private drivers," Olena says.

It's a dilemma that Mariupol's mayor, Vadym Boychenko, recognises. "Many buses of civilians go to Russian rather than Ukrainian territory," he told the BBC, over the phone. "From the beginning of war, [the Russians] didn't allow any way to evacuate civilians. It's a direct military order to kill civilians," he claimed.

Oleksandr and Olena's driver managed to get them from their filtration camp to the Russian-occupied city of Berdyansk - through "fields, dirt roads, narrow pathways behind all the checkpoints", Olena says, because they didn't have the proper documents to pass a Russian inspection.

They then spent three days looking for a route out before finding another driver who was willing to risk everything to get them to Ukrainian-controlled territory. He managed to get around 12 Russian checkpoints and safely deliver them to Zaporizhzhia. The couple then took an overnight train to Lviv.

"From filtration camps you can only escape using these risky local private drivers," Oleksandr says. "Fortunately, there are good people among them."

Arriving in Lviv on the same day were Valentyna and her husband Evgeniy. They also managed to flee Mariupol last week. They were boarding a coach to a smaller city in western Ukraine - desperate for safety after their ordeal.

The situation in Mariupol, they said, had become dire. They survived by sheltering in a basement, living of tinned goods, a handful of potatoes they grew in their garden, and water taken from the boiler.

The filtration process was speedy for them, says Valentyna, 58, perhaps because of their age and because Evgeniy has a disability. But it was far worse for younger people, she said.

"The filtration camps are like ghettos," she says. "Russians divide people into groups. Those who were suspected of having connections with the Ukrainian army, territorial defence, journalists, workers from the government - it's very dangerous for them. They take those people to prisons to Donetsk, torture them."

She and Evgeniy also say many were sent from the filtration camps to Russia. Sometimes people were told they were destined for Ukrainian-controlled territory, they say, only for the coach to head to Russian-held territory instead.

Like Oleksandr and Olena, Valentyna says it was only because of their driver that they managed to escape.

"When we finally [escaped] and saw the Ukrainian fighters and the flag, when we heard Ukrainian language, everyone in the bus started to cry," she said. "It was just unbelievable that we stayed alive and finally fled from hell."

Both couples have now escaped Mariupol, a city that has become a symbol of the resistance and the suffering of Ukraine after the Russian invasion. Now they face an uncertain future - just four of the 11 million Ukrainians displaced by the conflict.

"The city does not exist anymore," Valentyna says. "Even walls. Just huge piles of ruins. I could never have imagined such violence."