Fighting between government forces and rebels is continuing in the city of Aleppo
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.
Counting the disappeared in the real time of a conflict is extremely difficult. It’s almost impossible for outsiders to double-check claims independently.
Recent history shows that accurate counts of those kidnapped and abducted can only begin once a conflict has finished.
Activists in Iraq are still trying work out how many people were killed or went missing during the country’s peak years of violence after 2003. In Latin America, it took Chile almost 20 years to count the exact number of its disappeared during the military coup of 1973.
But the numbers currently suggested by Syrian opposition activists do give a sense of both the scale of the country’s conflict and the uncertainty surrounding the fate of many of its citizens.
“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.
Alice Jay, campaign director at Avaaz, said Syrians were 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”, and that each case must be investigated.
“The panic of not knowing whether your husband or child is alive breeds such fear that it silences dissent,” she said.
Other Syrian rights groups backed the allegations. Fadel Abdulghani, of the Syrian Network for Human Rights, estimated that 28,000 people had disappeared since unrest against the government of President Bashar al-Assad began last year.
Muhannad al-Hasani, of human rights organisation Sawasya, said the figure could be as high as 80,000.
“People are being snatched at night, on the street and when no-one is looking,” he said.
Muhammad Khalil, a human rights lawyer from the Syrian city of Hassaka, said the Syrian government had two reasons for carrying out the abductions: “To directly get rid of the rebels and activists, and to intimidate the society so that it won’t oppose the regime.”
Avaaz collected its statistics through a network of independent human rights lawyers and local activist groups in Syria.
The scale of the work and the current instability meant the organisation could not independently verify each disappearance, but it confirmed to the BBC that none of the detentions listed had been official arrests.
Most of the people Avaaz spoke to had personally witnessed a friend or relative being taken from home or the street
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, UN and Arab League envoy Lakhdar Brahimi is due to arrive in Syria on Saturday for talks with Foreign Minister Walid Muallem, Damascus told AFP news agency.
Mr Brahimi has proposed a truce over the Islamic holiday of Eid al-Adha, which starts on 25 October, to “allow a political process to develop”.
The Syrian government has recently indicated that it is interested in exploring a temporary ceasefire – and opposition groups have said they would match this.
Calls for the truce come as the conflict threatens to spill over Syria’s borders.
Turkey’s armed forces have several times returned fire across the border into Syria after Syrian mortar shells landed inside its territory.
Turkish TV reported further cross-border exchanges on Thursday morning. Our correspondent says smoke could be seen rising from the Syrian border village of Haram, while explosions and small arms fire could be heard.