Six Shi'ites were shot dead in Beirut on Thursday, in an attack on supporters of Hezbollah and its ally who were gathering to demand the removal of the judge investigating the explosion that ripped through the city's port last year.
The shooting, which took place on a frontline of Lebanon's 1975-90 civil war and evoked scenes reminiscent of that conflict, marks the deadliest civil violence in Beirut since 2008.
It also highlights a deepening crisis over the probe into the catastrophic August 2020 explosion that is undermining government efforts to tackle one of the most dramatic economic meltdowns in history.
The Iran-backed Hezbollah and its ally, the Shi'ite Amal Movement, accused the Lebanese Forces (LF), a Christian party that has close ties to Saudi Arabia, of mounting the attack.
Interior Minister Bassam Mawlawi said snipers had opened fire and aimed at people's heads.
The LF denied any involvement and condemned the violence, which it blamed on Hezbollah "incitement" against Judge Tarek Bitar, the lead investigator into the port blast, which killed 200 people, wounded thousands and devastated swathes of Beirut.
Coming after repeated warnings from Hezbollah and its allies that continuing Bitar's probe would split the country, the violence may create a pretext to shut down or shelve a further investigation into the explosion.
LF leader Samir Geagea, whose group had a powerful militia in the war, said earlier that the shooting was the result of uncontrolled weapons in society, saying civil peace must be preserved.
During the attack, local television stations broadcast footage of bullets bouncing off buildings and people running for cover. One of the dead was a woman who was struck by a bullet while in her home, a military source said.
At a nearby school, teachers instructed infant children to lie face down on the ground with their hands on their heads, a Reuters witness said. A lifeless body was dragged from the street by bystanders in footage broadcast by al-Jadeed TV.
The army said in a statement the gunfire had targeted protesters as they passed through the Teyouneh traffic circle located in an area dividing Christian and Shi'ite Muslim neighbourhoods.
The shooting began from the Christian neighborhood of Ain el-Remmaneh, from where the civil war was set off, before spiralling into an exchange of fire, a military source said.
Interior Minister Mawlawi said all the dead were from one side, meaning Shi'ites.
Hezbollah and the Amal Movement said groups had fired at protesters from rooftops, aiming at their heads in an attack they said aimed to drag Lebanon into conflict.
As Prime Minister Najib Mikati called for calm, the army deployed heavily in the area around Teyouneh and said it would open fire against any armed person on the road.
Bursts of gunfire were heard for hours.
The United States and France said the Lebanese judiciary needed to be allowed to investigate the port explosion in an independent and impartial manner.
"The Lebanese people deserve no less and the victims and families of those lost in the port blast deserve no less," U.S. Under Secretary of State Victoria Nuland said during a visit to Beirut.
"Today's unacceptable violence makes clear what the stakes are," said Nuland, in comments echoed by the French Foreign Affairs Ministry.
Judge Bitar has sought to question a number of senior politicians and security officials, including Hezbollah allies, suspected of negligence that led to the port explosion, caused by a huge quantity of ammonium nitrate and one of the biggest non-nuclear blasts on record.
All have denied wrongdoing.
Hezbollah has led calls for Bitar's removal, accusing him of bias.
On Wednesday, Geagea rejected what he described as any submission to "intimidation" by Hezbollah over Bitar, calling on Lebanese to be ready for peaceful strike action if the "other side" tried to impose its will by force.
The standoff over Bitar's investigation is diverting the newly formed government's attention away from addressing a deepening economic crisis, which has plunged more than three-quarters of Lebanese into poverty.
Though none of its members have been targeted by the probe, Hezbollah has accused Bitar of conducting a politicised investigation only focused on certain people.
These include some of its closest allies, among them senior figures in the Shi'ite Amal Movement who occupied ministerial posts, including former finance minister Ali Hassan Khalil who told al-Mayadeen TV this week the path of the probe threatened to push Lebanon "towards civil strife".
A court earlier on Thursday dismissed a legal complaint against Bitar, documents showed, allowing him to resume his investigation.
The violence is the worst since 2008 when followers of the Sunni-led government fought battles in Beirut with gunmen loyal to Hezbollah which took the streets until the government rescinded decisions affecting Hezbollah, including taking steps against a telecommunications network operated by the group.