At least two people have died and several have been injured, according to reports.
An Emergency Room doctor at Beirut's al-Sahel hospital who did not want to be named said the hospital has received two fatalities and nine injuries since the violence erupted.
Hundreds of supporters of Hezbollah and Amal movements were marching towards the Lebanese capital's Palace of Justice on Thursday when shots were fired at the protesters from an unknown location, forcing demonstrators and journalists to take cover, according to an army statement and local broadcasters.
There are multiple local reports of snipers shooting at demonstrators from the rooftops of buildings in the area. Local TV also showed a masked protester shooting a weapon from behind a street barrier and black smoke rising from one of the nearby buildings.
In a statement Thursday, Lebanon's military, which deployed extensively to the scenes of the gunfire, said it will shoot any armed person in the areas where the clashes are taking place and called for people to vacate the streets.