At least two people have been killed in Beirut following heavy clashes between rival Sunni Muslim gunmen.
The incident marks the latest violence fuelled by tensions over the uprising in neighbouring Syria.
Gunmen firing rocket-propelled grenades and machineguns clashed in the Lebanese capital's Tariq al-Jadideh district in some of the fiercest battles since sectarian fighting four years ago brought Lebanon to the brink of civil war.
The violence followed the killing of two members of a political alliance opposed to Syrian President Bashar al-Assad yesterday in the north of Lebanon.
Residents in the northern region of Akkar blocked roads and burned tyres to protest against the killing and demonstrations spread south to the main coastal highway and to Beirut, where several roads were cut off.
The fighting pitted gunmen from the Future Movement, loyal to anti-Syrian former Prime Minister Saad al-Hariri, against the pro-Syrian Arab Movement Party headed by Shaker Barjawi.
Two people were killed and 18 wounded. The security sources put the death toll at three - two of Mr Barjawi's supporters and one from the Future Movement - with nine people wounded.
Many of Lebanon's Sunni Muslims sympathise with Syria's Sunni-led uprising against Assad, whose father sent forces into Lebanon during its 1975-1990 civil war. The Syrian army finally pulled out in 2005 under international pressure.
Prime Minister Najib Mikati said: "The government is determined to continue to shoulder its national responsibilities amid this critical period in Lebanon and the region, and it will take all measures necessary to preserve civil peace."
