Heavy fighting claimed at least six lives and left dozens wounded in Lebanon's capital Thursday as an escalation of tensions around last year's massive portside explosion turned parts of Beirut into a war-zone.

The army deployed tanks and troops to quell street battles that sparked memories of the 1975-1990 civil war for a city already traumatised by last year's blast disaster and Lebanon's worst-ever economic crisis.

An armoured column of Lebanese troops deploys to Tayouneh, in the mainly Shiite southern suburbs of Beirut, after deadly violence erupts at a protest demanding the replacement of the lead investigator into last year's monster port blast
An armoured column of Lebanese troops deploys to Tayouneh, in the mainly Shiite southern suburbs of Beirut, after deadly violence erupts at a protest demanding the replacement of the lead investigator into last year's monster port blast AFP / JOSEPH EID

Bullets smashed into houses, while panicked civilians cowered indoors as the sound of gunfire and grenade blasts mixed with the wail of ambulance sirens for more than three hours.

The bloody unrest broke out after shots were fired at a demonstration by the Muslim Shiite Hezbollah and Amal movements.

Lebanese troops take up position in Tayouneh, in the mainly Shiite southern suburbs of Beirut
Lebanese troops take up position in Tayouneh, in the mainly Shiite southern suburbs of Beirut AFP / JOSEPH EID

The protesters were rallying against judge Tarek Bitar, tasked with investigating the massive explosion of poorly stored ammonium nitrate at Beirut's port that killed more than 210 people and destroyed swathes of the capital on August 4 last year.

The judge had in recent days been in the sights of Hezbollah and Amal after he subpoenaed top officials in his probe.

Supporters of Shiite groups Hezbollah and Amal demonstrate outside Lebanon's Palace of Justice demanding the replacement of the lead investigator into last year's monster Beirut port explosion
Supporters of Shiite groups Hezbollah and Amal demonstrate outside Lebanon's Palace of Justice demanding the replacement of the lead investigator into last year's monster Beirut port explosion AFP / JOSEPH EID

AFP correspondents said Thursday's violence started with sniper fire from residential buildings targeting the Hezbollah and Amal supporters, who returned fire with AK-47 assault rifles and rocket-propelled grenades.

President Michel Aoun called for calm in a televised speech late Thursday.

"Weapons cannot return as a means of communication between Lebanese parties, because we all agreed to turn this dark page of our history," he said in reference to the civil war.

A Shiite protester burns a portrait of US ambassador Dorothy Shea at a rally near the Palace of Justice in the Lebanese capital
A Shiite protester burns a portrait of US ambassador Dorothy Shea at a rally near the Palace of Justice in the Lebanese capital AFP / JOSEPH EID

He said political leaders were "heading towards a solution" out of the crisis.

A fighter of the Shiite Amal movement takes aim during clashes in Tayouneh in Beirut's southern suburbs
A fighter of the Shiite Amal movement takes aim during clashes in Tayouneh in Beirut's southern suburbs AFP / ANWAR AMRO

The army said it had responded to an exchange of gunfire in the Tayouneh - Badaro area as protesters headed to the Palace of Justice.

A Shiite activist armed with a rocket-propelled grenade launcher moves along a street in the Tayouneh area of Beirut
A Shiite activist armed with a rocket-propelled grenade launcher moves along a street in the Tayouneh area of Beirut AFP / -

It "raided a number of places looking for the shooters, and detained nine" people in total, including individuals from both sides. One was of Syrian nationality, it said.

The military did not specify who started the firefight.

Map locating the Tayouneh residential area in Beirut, where deadly chaos erupts during a rally against a port blast judge
Map locating the Tayouneh residential area in Beirut, where deadly chaos erupts during a rally against a port blast judge AFP / Jihad KACHAAMI

Interior Minister Bassam Mawlawi said the "exchange started with sniper fire, with the first casualty shot in the head".

He said at least six people were killed, all by gunfire, without specifying who fired the shots.

The health ministry said 32 people were wounded.

IMAGES A family runs holding their child as they flee the Tayouneh neighbourhood of Beirut during intensive gunfire clashes that broke out following a Beirut rally organised by the Shiite Hezbollah and Amal movements to demand the dismissal of the lead i
IMAGES A family runs holding their child as they flee the Tayouneh neighbourhood of Beirut during intensive gunfire clashes that broke out following a Beirut rally organised by the Shiite Hezbollah and Amal movements to demand the dismissal of the lead investigator into last year's port blast. AFPTV / Dylan COLLINS

Amal said three of its members were killed, while Hezbollah announced that funerals of two men and one woman would take place on Friday.

A doctor at Beirut's Sahel hospital earlier told AFP a 24-year-old woman was killed after she was hit in the head by a stray bullet inside her home.

Heavy fire rang out as ambulances rushed the wounded through the deserted streets, a few blocks from the Palace of Justice.

Iran-backed Hezbollah and Amal blamed the Lebanese Forces, a Christian party that is staunchly opposed to them, charging in a joint statement that the opposing side had "fired sniper shots with the aim to kill".

The Lebanese Forces denied involvement.

Political analyst Karim Bitar voiced concern about more trouble ahead.

"Hezbollah taking to the streets and throwing all its weight in this battle... could lead to big clashes and to the destabilisation of the entire country," he warned.

In the chaos, a limp body lying on a main street was carried away by rescuers as gunfire rained down around them.

Pictures shared on social media showed school children ducking under school desks.

Maha Yahya, director of the Carnegie Middle East Center, called the violence "a horrific reminder of unhealed wounds" from the civil war.

The man at the centre of the tensions, Tarek Bitar, is seen as a last hope for justice by many Lebanese but has been condemned as biased and corrupt by political leaders.

He has sparked deep divisions within the government between those who want to keep him and those who want him dismissed.

The protesters Thursday torched portraits of Tarek Bitar but also of US ambassador Dorothy Shea, charging that the judge is colluding with Washington, on the day senior American diplomat Victoria Nuland was visiting Lebanon.

Washington called for "de-escalation", while the UN Secretary-General Antonio Guterres reiterated "the need for an impartial, thorough and transparent investigation" into the port explosion.

But the investigator's fate is all but clear as Hezbollah and Amal press ahead with a campaign aimed at removing him.