Lebanon will observe a day of mourning on Friday for six people who died during clashes in Beirut on Thursday, Prime Minister Najib Mikati said.
"Lebanon is going through a difficult phase," Mr Mikati said. "We are like a patient in front of the emergency room.
"We have a lot of stages after that to complete recovery."
Unknown assailants killed six people and injured at least 32 others, Lebanon’s Health Ministry said, when shootings began before a Hezbollah-led protest near the Justice Palace to demand the removal of the judge investigating last year's port blast.
A woman who was struck by a stray bullet in her home in south-east Beirut was among the victims.
The clashes involved snipers, pistols, Kalashnikovs, hand grenades and rocket-propelled grenades.
By evening, calm had largely fallen.
Lebanon's Iran-backed Hezbollah and its ally, the Amal movement, accused the Christian Lebanese Forces (LF) party of carrying out the attacks on their supporters.
The violence signals growing unrest in Lebanon after powerful politicians spent the past year fighting judges investigating the Beirut blast probe amid a severe economic meltdown.
The LF denounced the accusation and demanded a thorough investigation into the incident, the state-run National News Agency reported.
The protesters "were subjected to an armed assault by groups from the Lebanese Forces party" stationed on nearby rooftops, Hezbollah and Amal said. Snipers were used to deliberately kill, they said.
Lebanese Forces' leader Samir Geagea denounced the incident and blamed it on the proliferation of weapons in the country.
“The main reason behind these events is the uncontrolled and widespread weapons that threaten citizens at all times and places,” he said. "I call on the president, the prime minister and the interior and defence ministries to launch thorough and accurate investigations to identify who's responsible for what happened today in the capital."
The Lebanese army was out in numbers in Tayouneh, Badaro, Cheyah and Ain al Remaneh districts in an attempt to restore calm and order to the streets. However, the clashes continued for hours until they stopped in the early evening.
Interior Minister Bassam Mawlawi confirmed that snipers were active at the site of the clashes.
"The first shots fired at the protesters were by snipers since they were aimed at the heads," he said. "This is a very dangerous sign. All the martyrs may be from one side and this might lead to possible reactions. I see it as an attack on the nation."
President Michel Aoun said the violence was a "painful and unacceptable scene, regardless of the causes and culprits," and said the events raised chilling memories of the deadly civil war from 1975-1990.
The conflict killed an estimated 150,000 and left thousands more missing. The militias involved then still play a major role in Lebanese public life today.
“It is not acceptable for weapons to return as a means of communication between Lebanese parties, because we have all agreed to turn over this dark page in our history," Mr Aoun told the nation in televised comments. He promised an investigation into the violence.
In the past week, Hezbollah and its allies have intensified a campaign for the removal of Judge Tarek Bitar from the investigation into the Beirut blast after he issued a warrant for the arrest of Ali Hassan Khalil, a former Amal finance minister, who failed to appear after a summons this week.
Men mostly dressed in black burned images of Mr Bitar in the streets, while a pick-up truck blasted chants calling the judge an “American traitor” after he summoned several politicians who are aligned with the group for interrogation.
“Sure they may be corrupt, but I am sure they have nothing to do with the Beirut blast,” Ihab Hamie, 32, told The National as he waved a banner condemning Mr Bitar.
People ran for cover and students in nearby schools hid under their chairs as shots and ambulance sirens could be heard in a live broadcast by Lebanon's Al Jadeed TV.
A resident of the Ain el Remmene district, close to the area where the clashes happened, said he and his family were “very scared”.
After two hours of hearing the exchange of gunfire, Marc, 37, decided to leave his apartment with his parents and his sister.
He said most people had also left the area.
“We decided to leave because my sister had a panic attack,” he told The National. "We were very scared."
A court earlier on Thursday ruled that Mr Bitar could resume his investigation into the August explosion that killed more than 200 people.
The Tayouneh area is located on the border between Christian and Shiite districts of Beirut, and was a front line in the civil war.
It is on the way from the predominantly Shiite southern suburbs of Beirut to the Justice Palace, where the protest was due to take place.
Hezbollah has accused Mr Bitar of conducting a politicised probe focused only on politicians close to the group.