Tougher international action is required to deal with a "grim catalogue" of state-sponsored torture and ill-treatment in Syria, a leading human rights group said.
Amnesty International said it had fresh evidence of widespread abuses of civilians detained by security forces in the year-long crackdown on anti-government protests.
In a report that identified 31 torture methods described by witnesses, Amnesty said the situation resembled the "nightmare" of Syria in the 1970s and 1980s.
At that time, the country was ruled by the father of today's president Bashar Assad.
The report was released as the United Nations announced that it would shortly deploy human rights monitors to neighbouring states to collect evidence of atrocities in Syria.
Access to the country itself is almost impossible for observers.
Amnesty based its conclusion on testimony gathered from Syrians in Jordan - including 25 who said they had been tortured.
"The testimony presented in this report, taken in the context of other human rights violations committed against civilians in Syria, is yet further evidence that torture and other ill-treatment in Syria form part of a widespread and systematic attack against the civilian population - carried out in an organised manner and as part of state policy and therefore amount to crimes against humanity," its report concluded.
At least 276 people had so far died in custody, it said, with security forces effectively given impunity to torture despite it being outlawed by a new constitution.
Amnesty's Ann Harrison said: "The experience for the many people caught up in the massive wave of arrests over the last year is now very similar to that of detainees under former President Hafez al-Assad - a nightmarish world of systemic torture.
"The testimonies we have heard give disturbing insights into a system of detention and interrogation which, a year after protests began, appears intended primarily to degrade, humiliate and terrify its victims into silence."
She said the International Criminal Court "represents the best option of securing real accountability for those responsible for the grave crimes that have been committed against people in Syria.
"But while politics makes that prospect difficult in the short term, Syrians responsible for torture - including those in command - should be left in no doubt that they will face justice for crimes committed under their watch. It is therefore essential that the Commission of Inquiry is strengthened and allowed to continue its work," she added.