* Gunfire erupted as people were en route to protest
* Hezbollah seeking removal of probe's judge
* Hezbollah blames Christian LF forces for killings
* LF denies involvement: PM calls for calm
* Shootings take place amid economic meltdown (Adds details from PM interview, link to Explainer)
By Maha El Dahan, Tom Perry and Laila Bassam
BEIRUT, Oct 14 (Reuters) - Tensions over a probe into last year's massive blast in Beirut burst into the worst street violence in more than a decade on Thursday, with six Shi'ites shot dead and gun battles reviving memories of the country's 1975-90 civil war.
Bullets bounced off buildings and people ran for cover during bursts of gunfire which lasted several hours on what was once a frontline in the war. At one school, teachers instructed infant children to lie face down on the ground with their hands on their heads, a Reuters witness said.
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 attacking its supporters, who were gathering to demand the removal of the judge investigating last year's port blast.
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.
The army initially said gunfire had targeted protesters as they passed through the Teyouneh traffic circle dividing Christian and Shi'ite Muslim neighbourhoods. It later said there had been an "altercation and exchange of fire" as protesters were on their way to the demonstration.
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 further investigation into the explosion.
President Michel Aoun vowed that those responsible for Thursday's gunfire would be held accountable, saying in a televised speech it was "unacceptable that weapons are once more the means of communication among Lebanese rivals".
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 hitting buildings and residents running for cover. One of the dead was a woman who was struck by a bullet while in her home, a military source said.
The shooting began from the Christian neighborhood of Ain el-Remmaneh, the site of a massacre that helped ignite the civil war, 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.
The army deployed heavily in the area around Teyouneh and said it would open fire against any armed person on the road. It later said it arrested nine people, including a Syrian.
Bursts of gunfire were heard for hours.
Prime Minister Najib Mikati told Reuters the 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," he said. "We have a lot of stages after that to complete recovery."
U.S., FRANCE URGE IMPARTIAL PROBE
The United States and France said Lebanon's judiciary must be allowed to investigate the port blast in an independent and impartial manner. Gulf state Kuwait urged its citizens to leave.
"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 from addressing a deepening economic crisis https://www.reuters.com/world/middle-east/lebanons-mikati-faces-tricky-path-safe-economic-ground-2021-09-13, which has plunged more than three quarters of Lebanese https://www.reuters.com/world/middle-east/why-is-lebanon-such-mess-2021-10-14 into poverty.
Mikati told Reuters that any minister who threatened to resign over the investigation into the port blast "should bear the responsibility of his decision", adding it was not the role of politicians to interfere in the judiciary.
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 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. Hezbollah took the streets until the government rescinded decisions affecting the group, including steps against a telecommunications network it operated.
(Reporting By Maha El Dahan, Alaa Kanaan, Laila Bassam, Mohamed Azakir, Tom Perry; Writing By Tom Perry; Editing by John Stonestreet, Samia Nakhoul and Daniel Wallis)
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