At least 28,000 people have disappeared in Syria since the start of an anti-regime uprising 19 months ago, arrested by troops or pro-government militiamen, a global activist group says.
Avaaz said the forced disappearances was part of a "deliberate" campaign by the authorities to silence dissent against the government of President Bashar al-Assad.
"Syrians are being plucked off the street by Syrian security forces and paramilitaries and being 'disappeared' into torture cells," Alice Jay, campaign director at Avaaz, said in a statement on Thursday.
"This is a deliberate strategy to terrorise families and communities," she said, adding that panic bred by forced disappearances helps silence anti-regime dissent.
Avaaz said at least 28,000 people have gone missing but that the number could be much higher.
"Human rights groups working inside Syria estimate that between 28,000 to 80,000 Syrians have been forcibly disappeared by the Assad regime over the past 19 months."
"The fate of each and every one of these people must be investigated and the perpetrators punished," said Jay.
"Nobody is safe."
Avaaz said it interviewed relatives of people who disappeared since the conflict erupted in mid-March last year and would hand over their testimony to the UN Human Rights Council, calling on the international body to investigate.
One woman, identified by Avaaz as Mais, said that she has concealed from her children the disappearance of her husband in the central province of Homs in February.
"The children need a father in their lives," Mais said. "They always ask me, 'Where is Dad? Who took him?' And I don't know how to respond. I have to lie to them. I tell them he is at work, that he is OK."
The sister of anti-regime protester Anas al-Shaghuri, 23, said he went missing in the coastal city of Banias in May last year and was reportedly handed over to security forces by someone he trusted.
He was tortured in detention, a fellow prisoners told the family, Avaaz reported.
Under international law, "the widespread or systematic practice of enforced disappearance constitutes a crime against humanity", said Avaaz.
Enforced disappearances in Syria are not new, according to Avaaz which says that some 7000 people who went missing during the rule of Assad's father and former president, Hafez al-Assad, are still unaccounted for.