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 warzone. 

The army deployed tanks and troops to quell the 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.

The bloody unrest, in which the sound of automatic gunfire and grenade blasts mixed with the wail of ambulance sirens, broke out after shots were fired at a demonstration by the Muslim Shiite Hezbollah and Amal movements.

The protesters were rallying against judge Tarek Bitar, tasked with investigating the massive ammonium nitrate explosion at Beirut's port that killed more than 200 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 in particular for insisting on subpoenaing top officials in his probe.

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.

"I can't handle these loud sounds, especially the RPGs," said one resident trapped in the combat zone in the city's southern Tayouneh area, who gave his name only as Samer. 

"It's the trauma of the Beirut blast coming back all over again."

Intense shooting

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 Lebanese Red Cross put the number of wounded at 30.

In the chaos, bullets smashed into houses and left craters in the walls of buildings, while many panicked civilians were trapped in their homes. 

Among those killed was a 24-year-old who was hit in the head by a stray bullet inside her home, a doctor at Beirut's Sahel hospital told AFP.  

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

The army made some arrests as they raided residential buildings looking for those behind the sniper fire, AFP correspondents said. 

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

Lebanese Forces chief Samir Geagea hit back, saying the real reason for the violence was the "widespread proliferation of arms," in reference to Hezbollah's arsenal.  

Geagea condemned the clashes and called on authorities to launch an investigation.

Political analyst Karim Bitar voiced concern about more trouble ahead, saying that "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".

Right to the truth

In the chaos, residents cowered in corridors away from windows, as some were shattered by the gunfire. 

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 desks and on the floor outside classrooms.

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

One social media user wrote that "Beirut now is similar to the Beirut of my childhood".

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

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

The protesters Thursday torched portraits of 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.

The Court of Cassation on Thursday turned down a lawsuit filed by two ex-ministers demanding Bitar's replacement, a court official told AFP on condition of anonymity. 

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

France meanwhile said it was "deeply concerned" about the hindering of the investigation and the violence.

"The Lebanese people are waiting for full light to be shed on the port explosion," said the foreign ministry. "They have a right to the truth."