At least six people died and dozens were injured as gunmen opened fire on protesters marching to commemorate the port explosion last year in which 219 people lost their lives.
Hezbollah, which organised the protest, says demonstrators were fired on by gunmen on rooftops.
They blamed a Christian faction, although the group denies the charge.
The protest was against the judge heading the inquiry into the blast but families of the victims support the judge.
