Human-rights groups working in Syria say at least 28,000 people have disappeared after being abducted by soldiers or militia.
They say they have the names of 18,000 missing since anti-government protests began 18 months ago and know of another 10,000 cases.
Online activist group Avaaz says nobody is safe from a deliberate government campaign of terror, the BBC reports.
Avaaz, which plans to give the UN Human Rights Council a dossier for investigation, has gathered testimony from Syrians who says husbands, sons and daughters have been forcibly abducted by pro-government forces.
"Syrians are being plucked off the street by security forces and paramilitaries and being 'disappeared' into torture cells," says campaign director Alice Jay.
Ms Jay says it's a deliberate strategy to terrorise families and communities: "The panic of not knowing whether your husband or child is alive breeds such fear that it silences dissent.
"The fate of each and every one of these people must be investigated and the perpetrators punished."
Figure put as high as 80,000
Fadel Abdulghani, of the Syrian Network for Human Rights, estimates that 28,000 people have disappeared since unrest against the government of President Bashar al-Assad began last year.
Muhannad al-Hasani of the Syrian human-rights organisation Sawasya put the figure even higher.
"According to information given to us by our contacts in villages across Syria," he says, "we think there could be as many as 80,000 forcibly disappeared people. People are being snatched at night, on the street and when no one is looking."
Muhammad Khalil, a human-rights lawyer from the Syrian city of Hassaka, says that although there are no precise figures, thousands of people have disappeared since March 2011.
"The regime is doing this for two reasons - to directly get rid of the rebels and activists, and to intimidate the society so that it won't oppose the regime," he says.
The Syrian government has so far not commented on the claims but in the past it has strenuously denied reports of human-rights abuses.
