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."