Saydnaya prison sits on a forbidding hill about half an hour's drive from the centre of Damascus.

In the last few days the entrance has been repainted in the green, white and black of Syria's revolutionary flag. The new colours did not dispel the sinister atmosphere of the place.

As I walked through the gates, I thought of the despair that must have gripped the thousands of Syrians who made the same journey.

One estimate is that more than 30,000 detainees were killed in Saydnaya in the years since the start of the Syrian war in 2011. That is a large proportion of the more than 100,000 people, almost all men but including thousands of women – as well as children – who disappeared without trace into Bashar al-Assad's gulag.

Other parts of Assad's prison system were less cruel. Phone calls home were allowed, and families were allowed to visit.

But Saydnaya was the dark and rotten heart of the regime. Fear of being consigned there and killed without anyone knowing what had happened was a central part of the Assad regime's system of coercion and repression.

The authorities did not have to tell families who had been incarcerated there. Allowing them to fear the worst was another way of applying pressure. The regime kept its boot on the throat of Syrians because of the power, reach and savagery of its myriad and overlapping intelligence agencies, and because of the routine use of torture and execution.

I was in other infamous prisons in the days after they were liberated, including Abu Salim, the former Libyan leader Colonel Gaddafi's notorious jail in Tripoli and Pul-e-Charki outside Kabul in Afghanistan.

Neither were as foul and pestilent as Saydnaya. In its overcrowded cells men had to urinate into plastic bags as their access to latrines was limited.

When the locks were smashed open, they left behind their filthy rags and scraps of blankets which were all they had to cover themselves as they slept on the floor. Torture and execution have already been documented in Saydnaya.

In the months to come it is certain that more information about the horrors perpetrated inside its walls will emerge from former inmates.

In Saydnaya's corridors you can see how hard it will be to mend the country Assad broke to try to save his regime. Now that the prison has been broken open, like the country, it has become a microcosm of all the challenges Syria faces since the Assad regime crumpled and was swept away.

One challenge is making a record of exactly what the regime did to its victims. In a sign of how far Syria has come in just a week, volunteers went to the prison to try to preserve Saydnaya's records.

Paperwork is scattered around offices and even on the concrete floor of the prison yard. Families pick up files and sheets of tattered documentation, trying to find a name, a date or a place that they recognise.

The disarray of the records looks as if someone tried to destroy what was done here in the name of Bashar al-Assad's Syria. When dictators and their henchmen fall, making sure they don't take the truth with them is a big part of a better future.

A musician called Safana Bakleh gave her group of volunteers face masks and blue nitrile gloves along with instructions about photographing and collecting documents.

Safana admitted they were amateurs and said they were taking matters into their own hands because the international human rights groups were not there, and evidence and documents were disappearing.

"Even if one family gets one answer that their loved on is not here anymore is deceased or he died in the hospital it is enough for me" Safana told me. "It is very chaotic. We don't where are the international originations supposed to be documenting all this chaos."

It is not just about families getting some release through at least knowing what happened to the disappeared. One day there might be trials of the perpetrators. Documents are evidence.

The truth the volunteers uncovered with their own eyes shocked them. All Syrians knew that the prisons were bad, but Saydnaya was much worse than they expected. Widad Halabi, one of the volunteers, took off her face mask and broke down in tears after an hour or so looking for evidence in the cell blocks.

"What I've seen here is a life not fit for humans. I imagined how they lived, their clothes. How did they breathe? How did they eat? How did they feel?

"It's terrible… terrible. There are bags of urine on the floor. They couldn't go to the toilet, so they had to put urine in bags. The smell. There's no sun or light. I can't believe people were living here when we were living and breathing our normal lives."

It will be hard for Syrians and their new rulers to track down the people they want to punish. Bashar al-Assad fled to Russia with his family. His brother Maher, with a reputation for violence and corruption as bad as anyone in his extended family, is thought to be in Iraq.

A couple of Assad's cousins ran into rebel fighters as they tried to escape to Lebanon. One of them, according to Reuters news agency, was killed in the resulting shootout.

When I entered Syria a week ago hundreds of cars full of despondent and scared families with some link to the regime, who believed they would be in danger in the new Syria, were leaving, queuing to get over the border into Lebanon. At the same time hundreds were driving in the opposite direction, desperate to get home.

Eventually there might be a legal process to prosecute Bashar al-Assad, members of his family and some of those who carried guns for the regime. Gathering evidence would be part of that. But the exodus in the last hours of the regime and in the confused days and nights that followed means that it will be hard to get to the people responsible.

At Saydnaya prison, families wander through the building, desperate for information, searching for those they've lost, horrified by everything they are seeing. Just being in Saydnaya's cells and corridors, freezing cold in December, reinforces a widespread desire to see the punishment of everyone implicated in the Assad regime's crimes.

A group of men gathered in the prison yard, smoking silently, some leafing through files that they had picked up off the ground. All those I spoke to said that the future must be built on justice for the past. The men in the group, all looking for missing, sons, brothers and cousins, called Saydnaya a mass grave. They want the head of Bashar al-Assad, literally. They murmured agreement when one of them said he had to be decapitated.

One of them, a young man called Ahmed, said he knew the brother he was searching for was alive because he could see him in his dreams. Ahmed himself had spent three years in Saydnaya.

"It was so bad, the torture, the food, everything. We were suffering."

Mohammed Khalaf, an older man, had been searching for his son Jabr since he was dragged from the family breakfast table by thugs from one of the state's intelligence agencies in 2014.

"We are many. People came from Qamishli, Hasaka, Deir al-Zour, Al Raqqa looking for our loved ones. Thousands are still in the streets looking for their children. It's not just me."

Inside one of the cell blocks, young men from Aleppo were warming themselves on a fire that they had lit in a metal tin, burning old prison uniforms that are scattered around every cell. They were looking for brothers who had been detained and then disappeared.

Like many others looking for information or a body at Saydnaya, the men had no money for a hotel. So they camped in the prison where they believe their brothers were consigned and most likely killed.

One of the men from Aleppo, Ezzedine Khalil, wants news of a brother taken by the regime on 1 September 2015. They all know the precise dates.

"We don't know if he is alive or dead. If he is dead, they should give us his body. They should tell us if he's dead. We just want to know. We want to know what to do next."

His friend Mohammed Radwan was looking for a brother and a cousin who were detained in 2012. Rumours were flying around that the night before the fall of the regime, 22 freezer lorries were brought to the prison to remove bodies. The rumours have not been confirmed but Mohammed and Ezzedine were convinced that they were true.

Mohammed looked exhausted and his anger flared up. He addressed himself to Assad.

"Where did you, pig, take the 22 fridge trucks? Everyone who took part in this crime, and everyone who served her in Saydnaya prison should be brought to justice. Everyone! Even if they were working in cleaning. They should all be brought to account."

"Because if they knew what was happening, at least they should have told the families of the prisoners that their dear ones were killed, slaughtered, hanged or tortured."

Both men ended with an Islamic prayer: "Allah is sufficient for me, and He is the best disposer of affairs."

Their hunger to see Assad and his men punished could become one of the drivers of events in the next few months. Syrians want to see their tormentors punished.

The extended Assad clan used Syria as their bank account. They helped themselves to stakes in businesses that might make profits. They controlled the lucrative market for telecoms and mobiles. As they raked in cash, Syrians struggled to eke out a living in an economy smashed by war and drained dry by rapacious and corrupt regime favourites. The new rulers of Syria have inherited big debts and an almost worthless currency. A couple of hundred dollars equals a plastic trash bag of bundles of Syrian pounds.

Corruption extended to the prison system. Victims and families desperate to avoid years in a hell-hole were prepared to pay big money to stay out.

Hassan Abu Shwarb served 11 years under sentence of death for terrorism, which was the word the Assad regime used for rebellion. Hassan, a quietly spoken man who is now 31, denies he ever joined an armed group. Instead, he says he was detained at a government office when he was getting documents necessary to apply for a passport so he could accept an offer to study in Canada.

His brother said the family paid a total of $50,000 (£39,509) in bribes on five separate occasions to try to get him out. In all cases the corrupt officials who had offered help for cash pocketed the money without releasing Hassan. A couple of weeks before the regime collapsed yet another corrupt judge offered to free Hassan for another $50,000.

After his arrest Hassan Abu Shwarb was tortured when he was detained for 80 days at a military intelligence interrogation centre. Among other injuries, the torturers broke one of his legs. Hassan says he was with one of his cellmates, a 49-year-old man, when he died after three days of torture. The jailors recorded death from a stroke.

Hassan was overjoyed to get home.

"When my mother held me after 11 years, I can't describe the feeling. There is nothing like going back to your home and neighbourhood."

But like many Syrians, Hassan's optimism about the future starts with determination that the fallen regime's leaders and acolytes should suffer for their deeds.

"They should be punished. We are human souls, not stones after all. And those who killed should be publicly executed. Otherwise, we won't get through this.

"We need to forget and move on. This is a happiness for all Syrians. We need to return to our work and responsibility to continue. We need to forget. We turned the page. All the sadness is behind us."

The leader of Hayat Tahrir al-Sham has started using his real name, Ahmed al-Sharaa, instead of his war time pseudonym, Abu Mohammed al-Jolani. The name change contains a message about looking ahead. The evidence is that Ahmed al-Sharaa will need to prioritise justice for the deposed regime if he does not want the chaos of people taking matters into their own hands.

The future is hard, and the past is full of pain. Here in Damascus, it feels as if a collective weight has been lifted from the shoulders of a nation.

Syrians know how deep their problems go. To preserve the optimism created by Assad's fall, Syrians want to see progress.