The group said it has contacted the families of some 2,918 people detained by security forces and compiled a list of names, the Lebanese Web site Yalibnan.com reported Thursday.
"It's shocking -- one person per hour is being disappeared. This is the face of the regime," Ricken Patel, executive director of Avaaz, told the Web site.
Nadim Houry, a Syrian researcher for Human Rights Watch, said incommunicado detention in the country is "widespread," the Web site reported.
Activists said Wednesday tanks and troops were deployed in areas around Damascus, scores were arrested and 11 killed, the Web site said.
The Local Coordination Committee activist network that documents protests also said security forces arrested 150 people in the Damascus suburb of Barzeh, Yalibnan.com said.