Tripoli has suffered sporadic sectarian violence since the Sunni Muslim-led uprising against Syrian President Bashar al-Assad broke out in March 2011, but after a night of mortar, grenade and machinegun fire, residents said this was the fiercest so far.
At least 18 people have been killed and more than 170 wounded in the Lebanese port city since the latest bout of fighting started on Sunday.
The use of heavy weapons stopped as the sun rose on Thursday and only the sound of sniper fire could be heard. The militant groups tend to fight through the night and rest by day.
Sunnis sympathetic to Syrian rebels live in a district of Tripoli overlooked by a hilltop neighborhood home to minority Alawites, the offshoot of Shi’ite Islam to which Assad belongs.
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 rebel-held Syrian border town of Qusair, where fighters from Lebanon’s Shi’ite Hezbollah movement are battling alongside Syrian troops.