The Syrian civil defence group known as the White Helmets says it is investigating reports from survivors of the country’s notorious Saydnaya prison that people are being detained in hidden underground cells.

Writing on X, the group says it has deployed five “specialised emergency teams” to the prison, who are being helped by a guide familiar with the prison’s layout.

Saydnaya is one of the prisons to have been liberated as rebels took control of the country.

Authorities in Damascus province reported that efforts were continuing to free prisoners, some of whom were “almost choking to death” from lack of ventilation.

The Damascus Countryside Governorate has appealed on social media to former soldiers and prison workers in Bashar al-Assad’s regime to provide the rebel forces with the codes to electronic underground doors.

They say they have been unable to open them in order to free “more than 100,000 detainees who can be seen on CCTV monitors”.

Video has been circulating online and through news outlets including Al Jazeera of what appears to be efforts to access lower parts of the prison.

In it, a man can be seen using a type of post to knock out a lower wall, revealing a dark space behind.

Other footage has shown prisoners being freed – including a small child being held with his mother. He is shown in a video of women being released that was posted by the Turkey-based Association of Detainees and The Missing in Saydnaya Prison (ADMSP).

“He [Assad] has fallen. Don’t be scared,” a voice on the video says, apparently trying to reassure the women that they were now safe.

Video verified by AFP showed Syrians rushing to see if their relatives were among those released from Saydnaya, where thousands of opposition supporters are said to have been tortured and executed under the Assad regime.

Rebel forces have swept across Syria, freeing prisoners from government jails as they went.

Throughout the civil war, which began in 2011, government forces held hundreds of thousands of people in detention camps, where human rights groups say torture was common.

On Saturday, the Islamist militant group Hayat Tahrir al-Sham (HTS) said it had freed more than 3,500 detainees from Homs Military Prison as the group took over the city.

The group was set up in 2012 under the name al-Nusra Front. It was allied to al-Qaeda but later severed ties – though the US, the UK and a number of other countries continue to see it as an affiliate of the jihadist organisation.

In 2016, the group took its current name HTS and later merged with other rebel factions. It is the most significant of several opposition groups taking part in this latest offensive.

As the rebels entered the capital hours later early on Sunday, HTS announced an “end of the era of tyranny in the prison of Saydnaya”, which has become a by-word for the darkest abuses of Assad’s era.

In a 2022 report, ADMSP said Saydnaya “effectively became a death camp” after the start of the civil war.

It estimated that more than 30,000 detainees had either been executed or died as a result of torture, lack of medical care or starvation between 2011 and 2018. Citing accounts from the few released inmates, at least another 500 detainees had been executed between 2018 and 2021, it said.

In 2017, Amnesty International described Saydnaya as a “human slaughterhouse”, in a report that alleged that executions had been authorised at the highest levels of the Assad government.

The government at that time dismissed Amnesty’s claims as “baseless” and “devoid of truth”, insisting that all executions in Syria followed due process.

Video cited by Reuters showed rebels shooting the lock off Saydnaya prison gate and used more gunfire to open closed doors leading to cells. Men poured out into the corridors.

Other footage, which the Reuters news agency says was taken on the streets of Damascus, appears to show recently freed prisoners running down the street.

In it, one asks a passer-by what happened.

“We toppled the regime,” they respond, eliciting an excited laugh from the former prisoner.

Of all the symbols of the repressive nature of the Assad regime, the network of prisons into which those expressing any form of dissent were disappeared cast the longest and darkest shadow.

In Saydnaya, torture, sexual assault and mass execution were the fate of thousands. Many never re-emerged, with their families often not knowing for many years whether they were alive or dead.

One of those who survived the ordeal, Omar al-Shogre, told the BBC on Sunday about what he endured during three years of incarceration as a teenager.

“I know the pain, I know the loneliness and also the hopelessness you feel because the world let you suffer and did nothing about it,” he said.

“They forced my cousin whom I loved so much to torture me, and they force me to torture him. Otherwise, we would both be executed.”

A Syrian human rights network estimates that more than 130,000 people have been subjected to detention in these conditions since 2011. But the history of these intentionally terrifying institutions goes back much further.

Even in neighbouring Lebanon, the fear of being disappeared to a Syrian dungeon was pervasive during the many years that Damascus was the dominant foreign power.

The deep hatred of the Assad regime – both father and son – that simmered under the surface in Syria was due in large part to this industrial-scale mechanism of torture, death and humiliation that was intended to frighten the population into submission.

For that reason, rebel factions in their lightning drive through Syria that toppled President Assad made sure in each city they captured to go to the central prison in each one and release the thousands held there.

The image of these people emerging into the light from a darkness that had shrouded some for decades will be one of the defining images of the downfall of the Assad dynasty.