At least 28,000 Syrians have gone missing since they were abducted by forces loyal to President Bashar al-Assad during the 19-month-old revolt, activist group Avaaz said on Thursday.
Quoting statements from Syrian human rights lawyers and families, Avaaz said security forces had set out to terrorise communities by grabbing people off the street and torturing them.
Other rights groups have also accused Syrian rebel groups of abducting people they deem pro-government.
Video footage accompanying the Avaaz statement showed two soldiers stopping a man and a woman, forcing them to the ground and dragging them away.
The film also showed an interview with a man saying his wife had been abducted six months ago in the Baba Amr district of Homs, which has been partly levelled by government bombardment.
"I lived my whole life with her," said the middle-aged man with his back to the camera, taking a long sigh and trembling.
"She disappeared. I have no hope. We searched everywhere for her. My children always ask me 'Where's Mum?'," he said.
One Syrian-based human rights organisation reported up to 80,000 people had disappeared, added Avaaz, a campaign network that aids opposition activists in Syria alongside other causes worldwide.
The figures could not be verified independently.
Activists say 30,000 people have been killed in Syria's conflict which started with peaceful protests but turned violent after months of oppression.
Another Syrian interviewed by Avaaz, Yousef, said his sister Zainab was abducted in Baba Amr six months ago. "If I know someone who was killed, I resign to God that they are dead. If I know someone who is injured, I still have hope they might heal," he said.
"But the unknown ... the only word that expresses it is 'unknown'."
Avaaz said it would hand over the reports of the abductions to the United Nations Human Rights Council, saying they amounted to "enforced disappearances".
Such abductions, where a state refuses to acknowledge a person has been taken, are seen as a crime against humanity under international law.