Louay Hussein was taken into custody at the Syria-Lebanon border as he was heading to Spain to visit his family, according to the Building the Syrian State party. It also said that he faces a number of "ready and tailor-made charges," including "weakening national sentiment and weakening the morale of the nation."
This is the second time Hussein, who is a longtime opposition activist and the leader of Building the Syrian State, has been detained since the uprising against President Bashar Assad began in March 2011. He also spent several years in jail in the 1980s.
There was no immediate comment from Syrian authorities, who rarely if ever confirm such arrests.
Human rights groups say the government has rounded up tens of thousands of Syrians, many of whom disappear in custody never to be seen again.
A UN panel last year accused Damascus of committing a crime against humanity by making people systematically vanish. Separately, Iran today denied that an Iranian nuclear scientist had been killed in Syria.
Syrian opposition activists and a pro-government website reported this week that four Syrian nuclear scientists were gunned down on a bus near Damascus. The Observatory had said an Iranian nuclear scientist also was killed in the attack.