BEIRUT — 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 Thursday.

Quoting statements from Syrian human rights lawyers and families, Avaaz said security forces had set out to terrorize 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.