Lebanese President Michel Aoun vowed that those who were responsible for the violence that erupted on Thursday in Beirut will be held accountable.
Speaking during a televised speech after the shootings that left six dead and dozens injured, Aoun said it was "unacceptable that weapons are once more the means of communication among Lebanese rivals".
Prime Minister Najib Mikati announced on Twitter that Friday would be a day of public mourning for the lives of those killed in the violence.
Mikati told Reuters that Thursday's events were a setback to the government but would be overcome.
"Lebanon is going through a difficult phase not an easy one. We are like a patient in front of the emergency room," Mikati said in an interview.
"We have a lot of stages after that to complete recovery."
Six Shiites were shot dead and dozens were left wounded, in an attack on supporters of Hezbollah and Amal movement who were gathering to demand the removal of the judge investigating last year's devastating Beirut port blast.
The exchanges of fire along a former front line from the 1975-90 civil war involved pistols, automatic rifles and rocket-propelled grenades, and were reminiscent of that conflict. Gunfire echoed in the Lebanese capital for several hours and ambulances, sirens wailing, rushed to pick up casualties. Snipers shot from buildings. Bullets penetrated apartment windows in the area.
Tensions had been running high after the Iran-backed Hezbollah and Amal demanded the removal of Tarek Bitar, the judge leading the investigation into the August 2020 port explosion.
Lebanon's army said on Thursday it arrested nine people, including a Syrian.
The army said on Twitter that it remained deployed in the area to ensure violence did not occur again.