Six people were killed and 30 others wounded in Beirut on Thursday at a demonstration by supporters of Hezbollah and the Amal movement against the lead judge investigating the massive blast in the city's port last year.

Violence erupted in the Lebanese capital as dozens of Hezbollah and Amal supporters protested outside the Justice Palace. They were exposed to bursts of fire in the nearby Tayouneh area, the Lebanese army said.

Heavy gunfire and two explosions from shells were heard by reporters at the scene.

Local media circulated video clips showing armed men firing in the streets amid reports of snipers in the buildings.

The demonstration coincided with the Civil Court of Cassation's rejection of a lawsuit filed by two former ministers requesting that Judge Tarek Bitar, who is leading the Beirut blast investigation, be removed from the case.

Bitar has sought the prosecution of high-level officials, some of whom have submitted complaints before multiple courts demanding that the case be transferred from Bitar's hand, forcing him to suspend the investigation twice so far.

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 of Beirut, AFP reported.

The army deployed heavily and sent patrols to the area to search for the gunmen.

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

(With input from agencies)