As they left their cells a toddler could be seen walking down the corridor, having apparently been held with his mother.

Tens of thousands of detainees have so far been freed from Syria's prisons, according to Rami Abdurrahman of the Syrian Observatory for Human Rights.

Syria's prisons were notorious during the Assad rule, highlighted in 2013 by a defector known as Caesar who smuggled out photos showing evidence of torture, disease and starvation.

At Sednaya, Amnesty and other groups claim dozens of people were secretly executed every week, estimating up to 13,000 Syrians were killed between 2011 and 2016.

Some may still be behind bars at the prison, with teams so far unable to break into hidden underground cells.

The civil defence force, known as the White Helmets, said their teams have begun a "mission" to access the lower layers.

"The teams consist of search and rescue units, wall-breaching specialists, iron door-opening crews, trained dog units, and medical responders," the group said.

"These teams are well trained and equipped to manage such complex operations."

Omar Alshogre, who was detained for three years and survived relentless torture, watched from abroad as videos showed prisoners fleeing.

"A hundred democracies in the world had done nothing to help them," said Mr Alshogre, a human rights advocate based in the US and Sweden.

"And now a few military groups came down and broke open prison after prison."

Among those freed from one of Damascus's prisons was writer Bashar Barhoum, who told the Associated Press he was due to be executed on Sunday.

"I haven't seen the sun until today," the 63-year-old said, after seven months locked up.

"Instead of being dead tomorrow, thank God, he gave me a new lease of life."