Syrian police have arrested the country's most prominent human rights lawyer Anwar al-Bunni, his family said.
A number of other people have been detained, in what rights groups are describing as the biggest crackdown for several years.
It came after hundreds of Syrian and Lebanese intellectuals signed a petition calling on Syria to recognise Lebanon as a fully independent country.
The letter was published in a Lebanese anti-Syrian newspaper last week.
It urges Syria to release all Lebanese political detainees and condemns political assassination.
Mr Bunni's family says he was dragged away by two men outside his house.
Michel Kilo was arrested on Monday
"Anwar asked them to show him an arrest warrant so he would go with them, but they forced him into the car, and drove away," his brother told the Associated Press.
Human rights groups say other arrests are likely to follow.
Mr Bunni's arrest follows the detention of prominent writer and activist Michel Kilo on Sunday.
Mr Kilo was summoned for questioning and not released, although there has been no official confirmation of his arrest.
Syria frequently arrests human rights activists and others who publicly criticise government policies.
The London-based humans rights group Amnesty International says hundreds of political prisoners are detained in Syrian jails, where they risk of torture.