Troops manned checkpoints and searched cars and houses in Tripoli's Bab al-Tebbaneh neighbourhood, a Sunni Muslim stronghold, and Jabal Mohsen neighbourhood, whose residents hail from the same Alawite sect as Syrian President Bashar al-Assad.
The communities' long-running feud erupted into violence on Friday after dozens of people took to Tripoli's Nour Square to show support for a three-month-old Syrian revolt that has drawn bloody crackdowns by Assad's government.
Security sources said between four and seven people, among them a boy and a soldier, were killed as street fighters attacked each other with assault rifles and grenades. At least 48 people were wounded.
In a statement, the army said it had "returned the situation to normal" and was enforcing a ban on residents carrying guns.
"The army leadership affirms (that) it will not be lax with those who toy with security and who were the reason behind both civilian and military casualties."
Assad's domestic challenges have been watched warily in neighbouring Lebanon, where sectarian tensions have often been inflamed by Syrian involvement.
Damascus's clout was especially strong during the Syrian military presence in Lebanon between 1976 and 2005, which also helped the standing of the small local Alawite community.
Since last month, northern Lebanon has seen an influx of Syrians fleeing an assault by Assad's forces on the border village of Tel Kelakh.