Damascus, Syria (CNN) -- It was a snowy January day in Damascus and 10-year-old Mada al-Zoabi was running downstairs to play in the snow - but then the sight of her father coming home shocked her.
"Dad? Is that you?" exclaimed a startled Mada, who did not recognize her father after not seeing him for nearly a month. "His hair was big and his face was like this," she said squeezing her cheeks in. "My mother told me he was traveling to another country, but I knew he was in jail," Mada said.
"We ran and hugged each other very tight," said her father Zaidoun Zoabi, an outspoken Syrian government critic.
Zaidoun said he spent 26 days in a regime detention facility, and was released looking like a "ghost." He said his health deteriorated and he lost more than 18 kilos (40 pounds).
"Food is one piece of bread in the morning and then they give you three olives. Every day we lose five to six people who die," Zaidoun told CNN in a recent interview in Damascus.
"They do not die under torture. Torture is there. I am not going to talk about physical torture, because this is something really trivial in comparison to torture to souls. Just imagine you cannot move for a single second for 24 hours, you are allowed to go to the toilet twice a day, they will count until 10 and then you have to leave the toilet," he said.
Zaidoun said he was crammed into a small room, 21 square meters (221 square feet) with 91 men. He compares it to being stuck in an elevator with a large group of people and little oxygen.
The 39-year-old human rights activist was detained in December. He credits media coverage of his case by CNN"s Anderson Cooper and others, in addition to pressure from the U.N. special envoy for Syria Lakhdar al-Ibrahimi on authorities, for his release.
"This is not a detention center, this is just a factory for madness and death... The main thing is you do not know when you are going to leave. You stay in a place, there is no real charge against you. They keep you for indefinite time. You just sit there and wait for the mercy of God so that you are out," Zaidoun said.
There is no way for CNN to verify conditions in the Syrian government's detention centers, but human rights groups have gathered dozens of testimonies similar to Zaidoun's.
His 22-year-old brother, Soheib, who was detained with him, remains in custody with little news on his condition.
"He is just the most peaceful person and the most innocent person you could ever meet. He is such a nice guy, like my mother was telling me... can you imagine he is still wearing the same clothes?" Zaidoun told CNN as his 63-year-old mother Iffat broke down in tears. "It's OK mama," a tearful Zaidoun said, while his six-year-old daughter Julia hugged her sobbing grandmother.
No one knows for certain how many Syrians are being held by the regime. The Syrian opposition recently called for the release of all detainees as a precondition for entering into negotiations with the government. They estimate the number at more than 160,000 detainees.
Syrian regime officials say the figure is exaggerated and that detainees "with no blood on their hands" are being released daily.
But activists dispute these claims and say the numbers are even higher.
"The only charge against him is that he is my brother," said Zaidoun who is pleading for Soheib's release.
"Arresting a person here in Syria does not need a real justification and if they have a justification, like my activism, then God knows when you are out. You could stay there for months, for years, nobody knows," he added.
Another activist and family friend, Keffah Ali Deeb was detained in August along with another close friend Rami Hinawy, who is still in detention.
Keffah, a 30-year-old children's writer, painter and activist spent 18 days in another Damascus detention facility. She was the only woman there and was kept in solitary confinement.
Keffah says she was not physically tortured, but she endured what she says was worse.
Everyday Keffah would hear the sound of people being tortured outside her door.
"Whenever I'd hear a voice, I would think its Rami. So I would jump on top of the sink in my cell and look through a small fenced opening to see if it was Rami being tortured..." Keffah recalled. She could not see much through it, but she was able to see men in their underwear being beaten up.
The story of one detainee in the cell next to her is one that Keffah says will haunt her forever.
"I was detained during Ramadan, and there was a detainee who clearly sounded like an old man. I used to hear him at certain times, knocking at the door saying "God help you, God bless you my son," begging them (the jailers) to just give him a drop of water because he was fasting," she said.
"Are you fasting you animal?" the jailers would respond."
Keffah says she watched as they would then take the man to a tap and let him drink -- but they would continuously slap him, beat him up and curse him while he drank the water. She would then hear the old man thanking them for giving him water.
"I would sit crawled up in a corner in my cell and cry so much thinking about what they were doing to this man," she said.
As she told the story whispering so no one could overhear in a Damascus café, Keffah was overcome with emotion.
"One of the worst forms of torture is the psychological one," she said as she wiped away her tears.
On the day of her release, Keffah recalls being taken down into a car. In the back was a blindfolded man with his hands bound.
As they were led out of the car and into the elevator, the guards removed the blindfold off the old and frail man.
Tears rolled down his cheeks as he realized his fellow detainee was a young woman.
Keffah sat in the room where an investigator met with them before their release.
The old man thanked the investigator.
"When he spoke I realized this was the same old man who would beg for water. I remembered his voice," she said.
As the investigator gave the man his belongings, he apologized: "Sorry old man, we apologize, we picked you up by mistake."
Keffah said crying: "All this! All he went through! And it was a mistake?! I will die and never forget his face."
Keffah and Zaidoun were among the tens of thousands of Syrians who took to the streets in 2011 in what began as peaceful anti-government protests.
But after a violent regime crackdown, some demonstrators began to take up arms.
Others defy government shelling and sniper bullets to provide humanitarian aid to those in need.
Keffah is an Alawite, the same sect as President Bashar al-Assad, but she refuses to be identified as that. "I am a Syrian," she says.
She denies that her country is being engulfed by a sectarian war. She says her group delivers aid and reach out to the different communities in an effort to keep Syrian society united against a sectarian rift.
Both Zaidoun and Keffah are opposed to any international military intervention in the conflict. They believe a negotiated end to violence is still possible and want a political process that would turn Syria into a democratic state.
But as the brutal civil war that has claimed nearly 70,000 lives continues with no end in sight , they say the plight of the tens of thousands of detainees is forgotten.
When asked how many people they know who are still detained, Keffah says: "It's a long list. A hundred faces are flashing through my head at this moment."
People are detained at random. Keffah and Rami were detained as they arrived to meet with a contact. Zaidoun was detained at a Damascus café after he showed up for a meeting.
Families of those detained spend weeks and months looking for their missing loved ones, not knowing if they are dead or alive. In some cases they get messages from released detainees.
Keffah says she spent her 18 days in jail trying to memorize the names of the dozens of detainees she said would be brought in every day.
In the facility where Zaidoun was held, he says the ages of detainees ranged from 13 to 75.
Torture is rampant and many they say are forced to confess crimes they did not commit.
In a press release last week, the International Committee of the Red Cross (ICRC) said they have only been able to carry out two prison visits to the central prisons in Damascus and Aleppo since 2011.
"That was good, but it is not enough. Of course, some places of detention are in areas too dangerous for us to enter. But this limited access means that there is no monitoring of the situation of detainees. That would be very worrying in any armed conflict and it is certainly a serious concern in Syria," the ICRC said.
But it is not only the regime that is holding detainees. Rebel fighters in opposition controlled areas have captured their own detainees too. With limited access amid a state of war, it is not clear how many people are detained by the Free Syrian Army and other rebel factions.
Zaidoun and Keffah know far too well the risks associated with speaking out, but they say fear of detention will not stop them.
"Of course I am afraid, but freedom tastes so good. No one, other than those who taste it for the first time, know what that is like. We have started a journey from which there is no turning back," Zaidoun said. "It will be a betrayal for those who lost their lives."
Keffah, whose name means struggle in Arabic, believes she has a duty to the next generation of Syrians.
"I don't want to take anything from this. I want the day to come when another generation has a better life than the one we had," she said.
"To me, my father, his generation and the one before his are all criminals because they were silent. Imagine us staying silent and leaving this for our children."