At least three people are reported to have been killed and many more injured as armed clashes broke out in Beirut during a protest against the lead judge investigating last year's massive blast in the city's port.

The exchanges of fire involving snipers, pistols, rifles and rocket-propelled grenades were a dangerous escalation of tensions over the domestic probe.

Gunfire echoed in the capital and ambulances rushed to the scene amid reports of casualties.

Four projectiles fell near a private French school, causing panic, a security official said. The students huddled in the central corridors with the windows open to avoid major impact, in scenes reminiscent of the 1975-1990 civil war.

The protest outside the Justice Palace was called by the powerful Hezbollah group and its allies who are demanding the removal of Judge Tarek Bitar.

It was not immediately clear what triggered the gunfire, but tensions were high along a former civil war front line between Muslim Shiite and Christian areas.

A journalist with the Associated Press saw one man open fire with a pistol during the protest. Another witness said he saw people shooting in the direction of protesters from the balcony of a building.

At least two men were seen injured and bleeding. The army deployed heavily in the area following the shooting.

In a statement, Prime Minister Najib Mikati appealed for calm and urged people "not to be dragged into civil strife".

Hundreds of tons of ammonium nitrates that were improperly stored at a port warehouse detonated on August 4 2020, killing at least 215 people, injuring thousands and destroying parts of nearby neighbourhoods.

It was one of the largest non-nuclear explosions in history and has further devastated the country already hit by political divisions and unprecedented economic and financial meltdown.

Mr Bitar, the second judge to lead the complicated investigation, has come up against opposition from Lebanon's powerful Hezbollah group and its allies who accuse him of singling out politicians for questioning, most of them allied with Hezbollah.

None of the group's officials have so far been charged in the 14-month-old investigation.

The armed clash could derail the country's month-old government even before it begins tackling Lebanon's unprecedented economic crisis.

A cabinet meeting was cancelled on Wednesday after Hezbollah demanded urgent government action against the judge. One Hezbollah-allied minister said he and other cabinet members would stage a walkout if Judge Tarek Bitar was not removed.