In a statement, nine local rights group said Wednesday that Ghassan al-Najjar was released Monday. There were no details on the health of the activist, who is in his mid-70s.
Al-Najjar leads a small group called the Islamic Democratic Current. He was arrested at his home in the northern city of Aleppo on Feb. 2.
The same groups had on Sunday expressed concern over his deteriorating health and urged authorities to release him immediately.
There was no official comment on al-Najjar's detention or release. An online campaign calling for protests on Feb. 5 fell flat when no protesters showed up in Damascus.