Abdelsalam al-Mismari was shot in the heart as he left a mosque after Friday prayers, the BBC reported Saturday.
Benghazi security spokesman Mohammed al-Hijazy said the killing appeared to be the work of a sniper.
Mismari was "a hero and a man of principle who did what he preached," said Hannah Ghallal, a colleague and friend.
He opposed the presence in Libya of the Muslim Brotherhood and was critical of armed groups that have refused to disarm after helping to topple Gadhafi.
In other attacks in Benghazi, a retired air force colonel was killed as he left a mosque and a police colonel was shot dead in his car, officials said.