The Syrian authorities are reported to have detained prominent activist and writer Michel Kilo, though there is no official confirmation of the arrest.
Syria is often criticised for its poor human rights record
His family said he was summoned for questioning by the security police on Sunday and has not been released.
A local human rights group said it expected Mr Kilo to be put on trial for signing a petition published in a leading anti-Syrian Lebanese newspaper.
The petition condemns political assassinations to silence dissent.
Hundreds of Syrian and Lebanese intellectuals signed the document, which was published last week in the Nahar newspaper.
It calls on Syria to release all Lebanese political detainees, while underlining the need to "respect and consolidate the sovereignty and independence of Lebanon and Syria" in the face of "Israeli aggression and American hegemony".
United Nations investigators have accused Syrian officials of involvement in the assassination of former Lebanese PM Rafik Hariri - an allegation denied by Damascus.
Syria frequently arrests human rights activists and others who publicly criticise government policies.
Humans rights group Amnesty International says several hundred political prisoners are detained in Syrian jails, where they risk of torture and ill-treatment.