At least five people have been killed in evening clashes in Tripoli between Lebanese fighters backing rival factions in Syria's conflict, security sources told Al Jazeera.
The fighting on Thursday brings the death toll in the new wave of sectarian violence that started on Sunday to at least 20 people. More than 200 others have been wounded.
The northern city has suffered sporadic clashes since the Sunni Muslim-led uprising against Syrian President Bashar al-Assad broke out in March 2011, but Wednesday night saw the the fiercest fighting so far, as warring sides used mortar bombs, grenade and machineguns.
Sunnis sympathetic to Syrian rebels living in the Tripoli district of Bab al-Tabbaneh have been fighting members of Assad's Alawite sect in the hilltop neighbourhood Jabal Mohsen.
The two groups have clashed in Tripoli on and off for decades, but the Syrian conflict has reopened old wounds, with each side accusing the other of using the city as a base for sending fighters and weapons in and out of Syria.
Syrian activists said the fighting in Tripoli flared after a fierce assault by Assad's forces on the Syrian border town of Qusayr, where fighters from Lebanon's Hezbollah group are backing government troops in their battle against rebels.
Local politicians said efforts to meet and discuss a ceasefire agreement have so far failed.
Footage of funerals for Hezbollah fighters killed in Qusayr was being watched by Shias across Lebanon, raising
sectarian tensions in other parts of the country, which suffered its own civil war from 1975 to 1990.
In the southern coastal city of Sidon, followers of a Sunni cleric blocked a funeral procession for a Hezbollah fighter.
Lebanese soldiers tried to break the blockade, leading to an exchange of gunfire between the protesters and security forces, residents said. No injuries were reported.