BEIRUT (Reuters) -At least six Shi'ites were shot dead in Beirut on Thursday, in an attack on protesters who were heading for a demonstration called by Hezbollah to demand the removal of the judge investigating the explosion that ripped through the city's port last year. 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, which took place on a frontline of the 1975-90 civil war. The shooting marks the worst civil violence in Beirut since 2008, and 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. LF leader Samir Geagea, whose group had a powerful militia in the war, condemned the shooting and said it was the result of uncontrolled weapons in society, saying civil peace must be preserved. In scenes reminiscent of the war, 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 before spiralling into an exchange of fire, a military source said. Interior Minister Bassam Mawlawi said snipers had opened fire and aimed at people's heads. All the dead were from one side, he said, meaning Shi'ites. Hezbollah and its ally, the Shi'ite 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. PM CALLS FOR CALM Hezbollah, a heavily armed group backed by Iran, has led calls for investigating Judge Tarek Bitar to be removed from the probe into the port blast, 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. 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, along with several explosions that appeared to be rocket propelled-grenades fired into the air, Reuters witnesses said. Political tensions have been building over the probe into the port explosion, which killed more than 200 people and devastated swathes of Beirut. The standoff over his 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. The judge has sought to question a number of senior politicians and security officials, including Hezbollah allies, suspected of negligence that led to the port explosion, which caused by a huge quantity of ammonium nitrate. All have denied wrongdoing. Though none of its members have been targeted by the probe, backed Hezbollah has accused Bitar of conducting a politicised probe 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. 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. (Reporting By Maha El Dahan, Alaa Kanaan, Laila Bassam, Mohamed Azakir, Tom Perry; Writing By Tom Perry; Editing by John Stonestreet and Samia Nakhoul)