Lebanon's army arrested 21 men suspected of involvement in sectarian fighting that killed 11 people in Tripoli over three days, a day after it took control of the northern city.

"In Bab el-Tebbaneh and Jabal Mohsen, army units arrested 21 people suspected of committing crimes, including shooting," the military said.

Residents of Jabal Mohsen, who share Syrian President Bashar al-Assad's Alawite faith, have fought frequently in recent months with inhabitants of the Sunni district of Bab el-Tebbaneh, who support Syria's anti-regime revolt.

"The military intelligence services have transferred eight (of the suspects) to military tribunals, while the rest are still in questioning," the army said in its statement.

The arrests come a day after the Lebanese authorities decided to place Tripoli, a city of some 500,000 people, under army control for six months.

Security measures such as patrols and checkpoints have already been increased in the port city, the military itself has announced.

While 80 percent of Tripoli's population is Sunni Muslim, the rest is Christian and Alawite, an offshoot of Shiite Islam.

Clashes rocked Bab el-Tebbaneh and Jabal Mohsen for three days but tensions were reduced by the military's takeover.

But according to a military source, gunmen opened fire Tuesday at an army convoy while it patrolled the aptly named Syria Street, which separates Jabal Mohsen from Bab el-Tebbaneh and acts as the frontline during clashes.

"Soldiers chased down the armed men through the streets of Bab el-Tebbaneh," the source added.

Former prime minister Saad Hariri, a key figure in Lebanon's Sunni-led opposition, called for Tripoli to be cleared of weapons.

On Monday, the decision to task the army with manning security in Tripoli was taken at a meeting between President Michel Sleiman, army chief General Jean Kahwaji and Prime Minister Najib Mikati.

Tensions between Bab el-Tebbaneh and Jabal Mohsen date back to Lebanon's 1975-1990 civil war but have been exacerbated by the conflict across the border in Syria, where Assad is battling a Sunni-led uprising.

Since 2008, there have been 19 outbreaks of sectarian violence between the two districts, killing more than 200 people and injuring some 3,000 others.