Saad al-Din Ghiyyeh, an official in the Islamic Action Front ( Jabhat al-‘Amal al-Islami) , was shot in his car by masked gunmen on a motorcycle.
He was rushed to hospital but died shortly afterwards.
Soldiers were deployed to the area to prevent further violence in the city, which has seen deadly sectarian clashes aggravated by the conflict in Syria.
Since 2012, at least 50 people have been killed in fighting between residents of the predominantly Alawite district of Jabal Muhsin and the neighbouring Sunni areas of Bab al-Tabbana and Qobbeh.
Syrian President Bashar al-Assad is an Alawite and members of the heterodox Shia sect dominate the government and security forces.
Syria’s majority Sunni community bore the brunt of the crackdown on dissent and is at the forefront of the armed revolt against the state.
Ghiyyeh, who survived an attempt on his life several months ago, was reportedly close to the head of the IAF, which has good relations with the Syrian government and Lebanon’s Shia Islamist movement, Hezbollah.
Two IAF members are said to have been arrested on suspicion of involvement in the bombings of two Sunni mosques in Tripoli in August that killed at least 42 people and wounded hundreds more.