At least six people were shot dead in Beirut this morning after a protest against the lead investigator into last year's devastating port blast turned violent.

Tank deployments and deadly exchanges of fire turned the city into a war zone during the demonstration organised by Shia parties.

Heavy fire rang out as ambulances rushed the wounded through the deserted streets, near the Palace of Justice, where hundreds of black-clad protesters had gathered moments earlier to demand Tarek Bitar's removal.

The judge has in recent days been in the sights of the Hezbollah and Amal parties in particular for insisting on subpoenaing top officials in his investigation into last year's deadly explosion in Beirut port.

The huge blast last August killed more than 200 people and destroyed large swathes of the capital.

According to Interior Minister Bassam Mawlawi, at least six people were killed today, all of them by gunfire.

They included a 24-year-old who was hit in the head by a stray bullet while inside her home, a doctor at the Sahel hospital in Beirut's southern suburbs told AFP news agency.

The Lebanese Red Cross put the number of wounded at 30.

AFP correspondents heard several loud bangs in the area, likely from heavy shelling, and saw plumes of black smoke rising from an apparent fire.

Residents cowered in corridors away from windows, as some were shattered by the gunfire.

Some scurried outside, running towards ambulances as civil defence volunteers tried to evacuate civilians, including young children.

Pictures circulating on social media showed children in a nearby school ducking under desks and gathering on the floor outside classrooms.

The army reported "bursts of gunfire in the area of Tayouneh - Badaro," a largely residential area of the capital.

"The army rushed to cordon off the area and deploy in its neighbourhoods and their entrance. Patrols started as did the search for the shooters to detain them," it said.

In a follow-up statement, the army warned it would open fire at anyone firing live rounds, calling on civilians to evacuate the area.

Prime Minister Najib Mikati called for calm to be restored and warned against attempts to drag Lebanon into violence.

In a statement, Hezbollah and Amal accused "armed and organised groups" of assaulting their supporters.

"They were exposed to direct fire from snipers... followed by intense shooting," they said in a joint statement.

They said the violence was deliberate and aimed to drag the country into violence, while calling on their supporters to remain calm and de-escalate.

Hezbollah's Al-Manar TV blamed the Lebanese Forces -- a Christian party that is staunchly opposed to the Iran-backed group.

AFP could not independently verify the claims.