Gunfire in Beirut killed at least five Lebanese Shi'ites in what authorities said was an attack on protesters who were going to take part in a demonstration called by Hezbollah to demand the removal of the judge investigating last year's port explosion, Reuters reports.

The shooting on a frontline of the 1975-90 civil war marks some of Lebanon's worst strife in years, and highlights a deepening crisis over the probe into the August 2020 blast that is undermining government efforts to tackle one of the most dramatic economic meltdowns in history.

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, adding that all the dead so far were Shi'ites.

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.

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.

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.