Fighting that erupted late on Tuesday in the northeastern Bab al-Tebbaneh and Jabal Mohsen districts of the port city died down early on Thursday when dozens of military vehicles deployed in the flashpoint areas.
The fighting, which claimed the lives of four people and left 58 wounded, had raged into the night despite a ceasefire that was supposed to begin at 8:00 pm (1700 GMT) on Wednesday.
"In order to put an end to the breach of residents' security, the army command has announced that it is reinforcing its presence in the sensitive areas," an army statement said.
"The army will confront those who fire first and calls on all parties to show calm and allow the military to take control of the situation," it added.
The deployment was slammed by some residents who blamed the military for standing by and doing nothing.
"Look at the soldiers -- they sit on tanks flexing their muscles but don't lift a finger," said Abu Mohammed, a resident of Bab al-Tebbaneh as a dozen armoured vehicles rolled past buildings riddled with bullet and rocket strikes.
Some were still burning and broken glass crackled underfoot as water poured from a burst main.
Militants armed with rockets, sniper rifles and grenades had battled in the streets on Wednesday, causing panicked residents to flee and shops and schools to close.
The dead included two brothers killed by snipers, a Palestinian nurse and a resident of the Jabal Mohsen district which is dominated by members of the Alawite community, an offshoot of Shiite Islam, which supports the opposition.
The latest unrest followed the eruption of similar firefights two weeks ago in Tripoli, Lebanon's second largest city after the capital Beirut, that killed nine people and wounded dozens.
It comes as Prime Minister Fuad Siniora continues trying to form a national unity government, efforts hampered by bickering between rival factions over cabinet posts.
Fighting centred on a main road separating the districts of Bab al-Tebbaneh-- where most residents are Sunni supporters of the Western-backed premier -- and Jabal Mohsen, a largely pro-opposition area.
Television footage showed masked gunmen running across deserted streets and smoke billowing from buildings as clashes continued intermittently throughout Wednesday.
Both sides announced later on Wednesday that they had agreed to observe a ceasefire from 8:00 pm and allow the army to deploy in the two neighbourhoods of Tripoli, a city of almost 400,000 people.
"The army will deploy to maintain security and prevent any armed presence," said a statement released after indirect negotiations between the two sides under the auspices of the Sunni mufti of north Lebanon, Sheikh Malek al-Shaar.
Not everyone was convinced.
"The army presence doesn't reasssure us -- we know from experience that it stands by meekly," said mother of eight Fawziya Mohammed Khodr, 51. "This is just a local anaesthetic."
Sporadic fighting has erupted despite a power-sharing deal between rival factions aimed at ending Lebanon's political crisis that boiled over into clashes that left 65 dead in May and raised fears of a return to all-out civil war.
Siniora has been struggling to form a new government despite the May 21 deal hammered out in the Qatari capital Doha between the Sunni-led ruling majority and the mainly Shiite opposition led by Hezbollah.
The premier said on Tuesday that he hoped a cabinet could be formed by the time President Michel Sleiman heads to Paris for a Mediterranean summit opening on Sunday.
The Doha accord allocated 16 cabinet seats to the parliamentary majority and 11 to the opposition, with Sleiman also naming three ministers.
