Prominent human rights lawyer Anwar al-Bunni said that all eight members of the Al-Atassi Forum for National Dialogue were taken from their home in dawn raids on Tuesday.
Al Bunni told Aljazeera that President al-Assad had allowed al-Atassi forum to continue its work and its sessions despite the Syrian security's crackdown on all other forums.
"That is why we were surprised by the arrest of its members," he said.
"We hope that this action was a mistake and would soon be retracted by releasing all the detainees in Syria."
Al-Bunni called on the international community to intervene to end human-rights violations in Syria.
"Syria is now in a very embarrassing situation in front of the whole world," he said.
"It is one of the few countries where human rights are openly violated and where people are held for free opinion and political reasons,"
"We appeal to the whole world, to all those who have a living conscience in the international community, in the Arab world and Syria, to call on and impose pressure on the Syrian authorities to release all detainees in Syria, particularly those who were arrested early on Tuesday."
"Human rights is an international issue which has sprung from an international human-rights agreement signed by all the world's countries," al-Bunni said.
The al-Atassi Forum was one of several political groups set up after President Bashar al-Assad took power in 2000 on the death of his father Hafez al-Assad.
But it was the only one to remain active after a crackdown by the authorities.
Forum founder Suhair al-Atassi and writer Hussein al-Awadat, who led regular debates, were among the detainees.
In other recent action against activists, Syrian intelligence officers on Sunday detained Muhammad Raadun, who heads the unofficial Arab Human Rights Organisation, after it took up the cause of jailed Islamist dissidents.
Dissident writer and rights activist Ali Abdullah was also picked up last week in his home in Qatana just outside the capital.
The Paris-based Arab Committee for Human Rights and other Arab and Syrian organisations have decided to send a delegation to Geneva, Strasburg, and Brussels to alert the Commission on Human Rights about detainees' circumstances in Syria.