Basim Faiz Mawat stood in the Damascus cell that his fellow prisoners used to call the "death dormitory", struggling to believe that the system that abused him for so long had been overthrown and his suffering had ended.

"I came here today only to see that truly nothing lasts forever," the 48-year-old said as he and another freed prisoner, Mohammed Hanania, visited the detention centre where their guards never showed mercy.

They were among thousands who spilled out of Syria's prison system on Sunday after a lightning rebel advance overthrew President Bashar al-Assad and ended five decades of his family's rule. Many detainees were met by tearful relatives who thought they had been executed years earlier.

"Every day in this room, which used to be called 'Steel 1 - the death dormitory,' one to three people would die inside every day," Hanania, 35, told Reuters.

“The sergeant was — when he didn't lose someone, when someone didn't die from weakness, he would kill him. He took them to the toilets and hit them with the heel of his shoe on their heads."

Hanania walked on past long rows of empty cells. Names of prisoners - Mohammed al-Masry, Ahmed and others - were scratched on walls with dates.

The floors were littered with rubble and discarded clothes. A row of blankets was still set out in one cell where prisoners had slept.

Both men looked up at an image on a wall of Assad, who is accused of torturing and killing thousands, abuses that were also rampant during his father Hafez's reign of terror.

"No one could have believed this would happen," said Mawat.