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.”