Syria faces allegations at the UN top court that it maintains a "pervasive" system of torture that has killed tens of thousands of people.
The case at the International Court of Justice (ICJ) marks the first time Damascus has faced international judges over the brutal civil war that started in 2011.
Victims have endured "unimaginable physical and mental pain and suffering" from "abhorrent treatment in detention, inhumane conditions of detention, and sexual and gender-based violence," the case brought by the Netherlands and Canada alleges.
Prisoners currently languishing in Syrian prisons are at "imminent risk of death or severe physical or mental harm," Canada and the Netherlands wrote in their submission to the court.
They also allege the widespread rape of women and children, mutilation, beheadings, plus the "particularly abhorrent" use of chemical weapons to intimidate and punish civilians.
"Urgent" and "legally binding"
Damascus has dismissed the case as "disinformation and lies," with a government source telling the SANA newswire in July the allegations "lack the slightest degree of credibility."
Canada and the Netherlands asked the ICJ to "urgently" demand Syria stop all torture and arbitrary detention, open prisons to outside inspectors and provide information to families about the fate of their loved ones.
The ICJ can take years to rule on a case, but urgent so-called "provisional measures" can be ordered in a matter of weeks and are legally binding.
An initial hearing was planned for July but postponed by Damascus.
Assad's return from isolation
The situation has gained renewed attention after the return of Syrian President Bashar al Assad to the international fold in May when he attended an Arab League summit.
Describing the hearings as a "watershed" case, Human Rights Watch hoped the ICJ proceedings would shine a spotlight on Assad's return from isolation.
Activist Helmi said there was no chance of "rogue state" Syria accepting any decisions by the ICJ, but insisted that any ruling was still important for victims and families.