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 people 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.
It intends to give the UN Human Rights Council a dossier for investigation.
The Syrian government has so far not commented on the claims but has in the past strenuously denied reports of human rights abuses.
Avaaz said it had gathered testimony from Syrians who say husbands, sons and daughters were forcibly abducted by pro-government forces.
They include Fayzeh al-Masri, from a suburb of Homs, whose 26-year-old son Ahmad Ghassan Ibrahim disappeared in February - the last number he called them from was traced to a military security branch.
The family were told by someone who answered his phone that he had died, but they have been unable to confirm this.
"We are certain that he would not have left us or his wife - who is expecting twins. We only want to know his fate," Mrs Masri told Avaaz.
The brother of Hussein Eisso, a 62-year-old Syrian-Kurdish activist, said he was taken from outside his home in Hasaka after attempting to stage a sit-in over the arrest of other activists.
He said his brother had since been moved between security branches, and had had serious health problems, including a stroke.
The BBC's James Reynolds, close to the Syrian border in Turkey, says it is often hard to establish real disappearance figures until a conflict is over, but the scale of the figures is an indication of the severity of the conflict in Syria.