Tension was running high in the northern Lebanese port city of Tripoli on Sunday, where a shop was torched and gunmen exchanged fire even as troops were out in force on the streets.

There were no immediate reports of casualties in the clashes, which erupted as dusk fell a day after a man died in a blast of unknown cause in an apartment block in the city.

Nine people were killed in sectarian clashes in the mainly Sunni city last week, adding to tensions in a deeply divided country where rival factions are still squabbling over the makeup of a new national unity government.

A security official told AFP that a tobacco shop run by an Alawite was set ablaze in the mainly Sunni district of Bab al-Tabbaneh, the scene of Saturday's blast in which 20 people were also wounded and several homes damaged.

Civil defense workers put out the fire before it could spread to other businesses, the official said, declining to be named.

The latest incidents took place despite the deployment over the past week of army and interior ministry forces in the port city since the deadly clashes between Sunnis and members of the Alawite community, an offshoot of Shiite Islam.

Armored vehicles were posted at entrances to Bab al-Tabbaneh and two other densely populated districts, Jabal Mohsen and Al-Qobbe, where the nine people were killed and 45 wounded in sectarian clashes on June 22 and 23.

It was still not publicly known on Sunday what caused Saturday's explosion, which killed a local resident with no known political affiliation.

Almost 500 people went to the victim's funeral which was also attended by dozens of armed militants, some wearing black headbands and chanting Koranic verses in praise of martyrdom.

Lebanon's Sunni mufti, Mohammed Rashid Qabbani, condemned the explosion as a "criminal act aimed at spreading sedition," and urged all politicians to help Prime Minister Fouad Saniora in his protracted efforts to form a new government.

Residents of Bab al-Tabbaneh who support the Western-backed majority in parliament have clashed repeatedly with Alawites in the nearby Jabal Mohsen district who back the Hizbullah-led opposition.

Also on Saturday, three grenades exploded in Bab al-Tabbaneh over a half-hour period but no one was injured and no buildings were damaged, security officials said.

And two men were wounded on Friday night in Tripoli when a grenade went off, an army spokesman said, adding that one lost a leg in the blast.

The Tripoli clashes have raised fears of a nationwide security breakdown amid protracted efforts by Saniora to form a national unity cabinet since a Qatari-brokered deal in May to end an 18-month political crisis.(AFP)