The fighting began in the country's second largest city overnight, when grenades exploded along a main road separating the neighboring districts of pro-government Sunni Muslims and Alawite supporters from the Hezbollah opposition.
Police and military units, who are at the scene, have failed to stop the fighting between crowds armed with small arms and grenade launchers.
The Bab el-Tabaneh and Jabal Mohsen districts were also caught up in fighting between opposing factions on June 22 when ten people were killed and 55 injured.
In May, Lebanon saw an end to an 18-month political crisis after the Western-backed coalition and the opposition reached a deal brokered in Qatar.
Prior to the deal, violent clashes that broke out between the factions in early May left more than 80 people dead and about 200 wounded.
Lebanon's political stalemate began in late 2006 when pro-Syrian ministers quit the cabinet of Prime Minister Fouad Siniora during a power struggle. The U.S.-backed government and the Hezbollah-led opposition spent over a year deadlocked and unable to elect a president.