(Reuters) - Gunmen clash in deadly street battles, protesters block roads with burning tires and opposition politicians demand the prime minister´s downfall, denouncing the army as an agent of a foreign power.

Fragile Lebanon´s sectarian tensions, which festered for two decades since the end of its ruinous civil war, have been re-ignited by the turmoil in powerful neighbor Syria and threaten to plunge the country into a sustained period of unrest.

In the northern city of Tripoli, where Sunni Muslims strongly support the 14-month uprising against Syria´s President Bashar al-Assad, nine people were killed in clashes last week triggered by the arrest of an anti-Assad activist.

The violence spread to the capital on Monday when Sunni gunmen fought street battles in a Beirut neighborhood following the killing of a Sunni cleric, also opposed to Assad, by Lebanese soldiers at an army checkpoint in the northern Akkar province.

On Tuesday, angry Shi´ites blocked roads in southern Beirut in protest against the abduction in northern Syria of a dozen Lebanese Shi´ites by rebels from the overwhelmingly Sunni Muslim insurgency against Assad.

"We are entering a phase of protracted instability in Lebanon. There is no direct way in which these events will be fully contained," said Eurasia Group analyst Ayham Kamel.

The Syrian uprising forced Lebanon´s Prime Minister Najib Mikati into a near-impossible balancing act between diehard supporters and opponents of Assad in a country which was long dominated by Syrian military power.

Mikati himself embodies some of the conflicting loyalties. He is a Sunni Muslim from Tripoli, but has close ties with authorities in Damascus and only came to power last year after Shi´ite Hezbollah and its Christian allies toppled Lebanon´s unity government, headed by Sunni leader Saad al-Hariri.

He has promoted a policy of "disassociation" from Syria´s troubles in a possibly doomed effort to insulate his country from the unrest across Lebanon´s only open land border.

Fighting has spilled over the frontier several times, along with several thousand Syrian refugees whose presence in northern Lebanon has helped inflame Sunni anger against Assad. Weapons smuggling is common and Syria has accused Lebanon of "incubating terrorist elements" it blames for the unrest.

Lebanon´s army last month seized and displayed three freight containers filled with Libyan weapons suspected of heading for Syria´s rebels, highlighting both the scale of arms shipments and efforts by authorities to curb them.

"There is no right way to balance these delicate interests, Kamel said. "It´s going to be very difficult for the government to appease the Sunni community and at the same time monitor or restrict arms flows to Syria."

EMBOLDENED ISLAMISTS

Hezbollah, an ally of Assad which has two ministers in Mikati´s cabinet, "wants to prevent any real effective support to the Syrian opposition that would tilt the balance on the ground", said International Crisis Group analyst Sahar Attrache.

But attempts to restrict support for the Syrian rebels have antagonized the increasingly emboldened Sunni Islamist activists and militants in northern Lebanon, where many complain about years of neglect by politicians in Beirut.

Hariri´s year-long absence from Lebanon, ostensibly for security reasons, has added to a Sunni political vacuum which has encouraged other Sunni groups to flex their muscles.

Last week´s violence in Tripoli was triggered by the arrest of Shadi al-Moulawi, an Islamist supporter of the Syrian uprising who was charged with membership of a terrorist group. Moulawi, who denies the charge, was released on bail of $330 on Tuesday in an attempt to defuse the escalating tensions.

"The showdown after Moulawi´s arrest is new. Before, these groups needed political cover but this time they are getting more independent of the political leaders," Attrache said.

The clashes in Tripoli pitted Sunni Muslim gunmen against Alawites - members of the same minority as Assad - but also against Lebanese troops when the army tried to end the fighting.

That military intervention, coupled with the killing of a Sunni Muslim cleric and his companion at an army checkpoint on Sunday, have stirred up anger against soldiers.

A Sunni Muslim parliamentarian accused the army of being "at the service of Syria" and some Tripoli residents heckled soldiers as traitors.

"Questioning the army is very worrying, very dangerous," Attrache said, adding that erosion of army legitimacy was one of the factors which contributed to Lebanon´s descent into 15 years of civil war in 1975.

The composition of Lebanon´s army broadly reflects the wide sectarian elements of Lebanon´s diverse Muslim and Christian communities, although many recruits come from the relatively poor northern Akkar province.

SHADOW OF CIVIL WAR?

Four Gulf Arab nations - Kuwait, Qatar, Bahrain and the United Arab Emirates - have told their citizens to stay away from Lebanon, citing security concerns, and Saudi Arabia´s King Abdullah said the "shadow of civil war" hangs over Lebanon.

The crisis is likely to hurt the summer tourism season, an important source of foreign revenue, and has helped push Beirut´s stock market index down to 1,154 points on Wednesday, 30 percent down from levels in April 2010 before domestic and regional unrest hit investor confidence.

Amid the growing fears of instability, Hezbollah - whose supporters seized control of west Beirut four years ago in the last bout of major violence in the capital - has sought to calm the fevered atmosphere.

Its leader Sayyed Hassan Nasrallah called for restraint on Tuesday after the kidnapping of 13 Lebanese Shi´ites in Syria, and avoided getting sucked into the fighting in Beirut on Monday which security sources said pitched supporters of Hariri´s Future Movement against a Sunni group sympathetic to Hezbollah.

"Hezbollah is going to sit on its hands. It is not going to do anything. Hezbollah´s modus operandi has always been not to get dragged into sectarian strife," said Amal Saad Ghorayeb, author of a book on the Shi´ite movement.

"So long as these groups don´t pose a threat to Hezbollah´s resistance (to Israel) in any way, there is no way Hezbollah will get dragged in just because of killings."

But it might still seek to use its influence with pro-Syrian groups in the north if it felt support for the Syrian opposition becomes "more aggressive", Attrache said.

A political source in Lebanon´s March 8 coalition, which includes Hezbollah, said the Shi´ite group would not resort to arms even if tensions in the north endured for decades. But he made clear Hezbollah was monitoring the region closely.

"There are serious efforts to turn the north into an operations room for Syrian rebels," he said. "This will only happen in the absence of the army and state authority."

Analyst Kamel said clashes were likely to continue in Tripoli between Sunni Muslim and Alawite gunmen, as well as fighting between Sunni Islamist groups and the army, although tensions in the capital itself might be contained.

"This is not a chapter heading to civil war, but rather one where conflicts or violence are geographically localized, particularly in the north," he said.

(Additional reporting by Laila Bassam; editing by Philippa Fletcher) (© Thomson Reuters 2012. 05/23/12)