The clashes between Sunni Muslim militants and Alawites, a Shiite sect, erupted at dawn when a hand grenade was thrown toward a Sunni area.
The fighting broke out in two of the city's poorest neighborhoods - Jabal Mohsen, populated by Shia Alawites who support the opposition, and Bab Tibbaneh, where the predominantly Sunni population backs the parliamentary majority.
The sides exchanged fire using assault rifles and rocket-propelled grenade launchers, while snipers shot at neighboring districts. Sniper fire forced the army to close the highway linking the port city of Tripoli and Syria.
The LBCI TV channel reported that the army had blocked the main roads into the neighborhoods, but had not entered the conflict zone.
The two districts saw a major clash earlier this month between supporters of Lebanon's Sunni-led governing coalition and an opposition alliance led by Shiite Hezbollah, which left 14 people dead and more than 100 injured.
The Tripoli fighting has cast a shadow over the unity government deal that ended a bitter political conflict between the two rival political forces. The new government, led by Prime Minister Fuad Saniora, was formed on July 11.