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.
Avaaz has gathered testimony from Syrians who says husbands, sons and daughters have been forcibly abducted by pro-government forces.
Alice Jay, campaign director at Avaaz, said: “Syrians are being plucked off the street by security forces and paramilitaries and being ‘disappeared’ into torture cells.
“Whether it is women buying groceries or farmers going for fuel, nobody is safe.”
She said it was 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,” she said.
“The fate of each and every one of these people must be investigated and the perpetrators punished.”
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“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”
Human rights lawyer Muhammad Khalil
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, we think there could be as many as 80,000 forcibly disappeared people,” he said.
“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, said 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 said.
Ceasefire hopes
The Syrian government has so far not commented on the claims but it has in the past strenuously denied reports of human rights abuses.
The UN says more than 18,000 people have been killed in the conflict with 170,000 fleeing abroad and 2.5 million in need of aid within the country. Opposition and human rights activists put the death toll at more than 30,000.
As violence continues, the Syrian government has recently indicated that it is interested in exploring a temporary ceasefire proposed by UN and Arab League envoy Lakhdar Brahimi.
Mr Brahimi wants a truce over the Islamic holiday of Eid al-Adha, which starts on 25 October, to “allow a political process to develop”.
Opposition groups have said they would match any government ceasefire.
Calls for the truce come as the conflict threatens to spill over Syria’s borders.
Turkey’s armed forces again returned fire across the border into Syria after a Syrian mortar shell landed just inside Hatay province on Wednesday.
