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This is the second night of violence to grip the city, threatening to crack the country's dangerous fault lines amid a political deadlock.

Heavy gunfire was witnessed around Cola Bridge in Beirut, Reuters reported quoting local news agencies, with no injuries reported.

The southern town of Tyre saw Hezbollah and Amal supporters tear up protest tents and set them on fire, at which point security forces intervened.

For weeks, the Lebanese security forces have taken pains to protect anti-government protesters, in stark contrast to Iraq, where police have killed more than 340 people over the past month in a bloody response to similar protests.

The two nights of violence - some of the worst since protests against the country's ruling elite began last month - gives a preview into a worst-case scenario for Lebanon's crisis.

Demonstrators demanding a complete government overhaul have stayed mobilised since protests began on 17 October, but a bitterly divided political class has yet to find a way forward.

Frustrated by the stalemate, protesters had called for roadblocks and a general strike on Monday, but an attack by supporters of Hezbollah and Amal on Sunday night weakened the turn-out.

By attacking protesters Sunday night, Hezbollah sent a message that it is willing to use force to protect its political power.

Confronting the powerful Iranian-backed Hezbollah, however, is out of the question for the military as doing so would wreck the neutral position it seeks to maintain and could split its ranks.

"The army is in a difficult position facing multiple challenges and moving cautiously between the lines," Fadia Kiwan, professor of political science at Saint Joseph University in Beirut, told AP.

She said the military has sought to protect the protesters and freedom of expression but is increasingly grappling with how to deal with road closures and violence.

The UN Security Council urged all actors in Lebanon on Monday to engage in "intensive national dialogue and to maintain the peaceful character of the protests" by respecting the right to peaceful assembly and protest.

Calling this "a very critical time for Lebanon," the UN’s most powerful body also commended Lebanon’s armed forces and state security institutions for their role in protecting the right to peaceful assembly and protest.

'This is Lebanon, not Iran'

Sunday night's clashes brought into full display the political and sectarian divisions that protesters have said they want to end.

"Shiite, Shiite, Shiite!" Hezbollah supporters waving the group’s yellow flag shouted, taunting the protesters, many of them Christians.

The protesters chanted back, "This is Lebanon, not Iran," and "Terrorist, terrorist, Hezbollah is a terrorist" - the first time they have used such a chant.

The violence began when supporters of Hezbollah and the other main Shia faction, Amal, attacked protesters who had blocked a main Beirut thoroughfare known as the Ring Road - a move the protesters said was aimed at exerting pressure on politicians to form a new government after Prime Minister Saad Hariri offered his resignation 29 October.

Carrying clubs and metal rods, the Shia movements followers arrived on scooters, chanting pro-Hezbollah slogans. They beat up several protesters. Both sides chanted insults, then threw stones at each other for hours.

Security forces stood between them but did little to stop the fighting. Finally, after several hours, they fired tear gas at both sides to disperse them. The road was eventually opened before daybreak Monday.

By that time, protesters' tents were destroyed in areas close to the Ring Road. The windshields of cars parked near Riad Solh Square and Martyrs Square - the central hubs of the protests - were smashed as were the windows of some shops.

'Dangerous phase'

The nationwide protests have so far been overwhelmingly peaceful since they started mid-October.

Politicians have failed to agree on a new Cabinet since Hariri’s government resigned. Hezbollah and Amal insist Hariri form a new government made up of technocrats and politicians, but Hariri - echoing protester demands - says it must be made up only of experts who would focus on Lebanon's economic crisis.

As the deadlock drags on, tempers are rising.

"The situation is moving toward a dangerous phase because after 40 days of protests, people are beginning to get tired and frustrated and might resort to actions that are out of control," Kiwan said.

One person has been killed by security forces during the protests, while six have died in incidents related to the demonstrations.

In the latest, a man and his sister-in-law burned to death Monday after their car hit a metal barricade erected by protesters on a highway linking Beirut with the country’s south.

Hezbollah issued a blistering statement Monday condemning the road closure, painting the protests as a danger to the country.

It called the deaths the result of "a militia attack carried out by groups of bandits who practice the ugliest methods of humiliation and terrorism against people".

In the increasingly tense atmosphere, "the role of the army is getting bigger", Kiwan said.

Hisham Jaber, a retired Lebanese general who heads the Middle East Center for Studies and Political Research, said the army is in a "delicate" position and could not have done more than it did Sunday night.

Army divisions

The military is already at the center of a debate in US policy-making circles.

The Trump administration is now withholding more than $100 million in US military assistance to Lebanon that has been approved by Congress, without providing an explanation for the hold.

That has raised concerns among some in the US security community who see the aid - largely used to buy US-made military equipment - as key to countering Iran’s influence in Lebanon.

Others, however, including pro-Israel lawmakers in Congress, have sought to defund the military, arguing it has been compromised by Hezbollah, which the US designates as a terrorist organisation.

US administrations have long believed that a strong Lebanese army could be a counter to Hezbollah's weapons and could deprive the militants of the excuse to keep their arms.

The 70,000-strong force split along sectarian lines during Lebanon’s 1975-90 civil war. Since then, it has largely succeeded in achieving a level of stability by maintaining a tough balancing act that includes coordinating with Hezbollah on security matters.

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Jaber said it is impossible for the security forces to clash with Hezbollah because "this will lead to divisions within the army".

"Hezbollah is a main part of the Lebanese people," he said. "Getting the army into a battle with them would lead to pulling away part of the Lebanese army, and this could be followed by other groups splitting from the army."

"The Lebanese army is the pole of the tent. If the pole collapses, the whole country will collapse. It is the duty of the army to protect state institutions."