But one of the worst horrors he recalls came when his guards started spraying insect repellent around the cell.
"Cockroaches started coming out of everywhere," Masri says. "The cockroaches started to walk on our faces. The wardens put them inside our clothes."
Masri is one of thousands of former detainees in Syria's sprawling prison underworld. He describes a litany of torture that has been not only barbaric but also systematic.
Survivors and lawyers say there are now more than 100 detention centres holding around 200,000 people jailed since the revolt against President Bashar al-Assad began in 2011.
According to the Syrian Observatory for Human Rights, up to 12,000 people have died in these prisons.
Mohammad Samaan, a 33-year-old activist from Damascus, remembers reading George Orwell's "1984", the novelist's dystopian vision of life under an all-knowing dictatorship, in the days before the uprising.
"When I became a detainee myself, I discovered that there is such a world, and it is in Syria," he says.
"Nothing, no amount of reading or listening to other people's stories could have prepared me for the horror of detention," adds Samaan, who now lives in Beirut.
Speaking quietly as he pulls on a cigarette, Samaan says he was jailed twice for activism against Assad's regime, enduring physical and psychological torture on both occasions.
An interrogator at one of Damascus's feared security branches told him: "We torture people because we are sadists. We enjoy torturing people."
"He electrocuted me and told me to write down everything I knew. He tried all he could to break me. I have never been so terrified in all my life," the brown-haired Samaan says.
'NOTHING IS ARBITRARY'
Masri, 36, says he also endured psychological torture.
"They would insult my wife, and they would tell me they would go to the house and rape her," he recalls.
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At the scores of security offices where detainees are usually first held for questioning, food, water and medication shortages are particularly extreme.
Round-faced Masri weighed more than 100 kilos (220 pounds) when he was first detained. By the time he was released he had lost more than half his body weight.
Like most detainees, Samaan and Masri -- who spoke to AFP on condition their names were changed -- were transferred from secret detention in Damascus to the infamous Adra and Saydnaya jails after trials they dismiss as farcical.
Masri's case was heard before a military court despite his non-violent activism.
"The regime doesn't respect its own laws when it comes to the detainees," says one Syrian human rights lawyer, speaking on condition of anonymity for fear of repercussions.
"There are four security agencies in Syria, and each does all it can to prove it is more brutal than the next."
The lawyer described a hellish network of jails, security offices and secret detention centres across the country.
"In Damascus alone there are 30 to 40 security offices, which are illegal, as well as an unknown number of secret detention sites," he says.
'MEMORIES HAUNT ME'
In June, Assad issued an unprecedented amnesty covering tens of thousands of people detained throughout the conflict under Syria's notorious anti-terrorism law.
But only a handful of high-profile prisoners of conscience have been freed.
Human rights activist Sema Nassar says the regime refuses to release peaceful activists who played a key role in the 2011 uprising because "it fears the impact" they might have if freed.
Syria's conflict began as a peaceful movement demanding democratic change, but evolved into a civil war after the regime unleashed a violent crackdown against dissent.
Activists say most of those who led the uprising are now dead, in jail, or missing.
Those who did survive detention say they can never forget what they endured.
"Memories come back to haunt me every day, when I eat, when I sleep," says Samaan. "It's so awful in there. Some things you just can't talk about."