Seen by many as neutral, the army has emerged as an arbiter
Heavy fighting has broken out again in the northern Lebanese city of Tripoli between supporters of the government and the Hezbollah-led opposition.
Explosions reverberated throughout the city as both sides fired machine-guns, mortars and rocket-propelled grenades in the latest outbreak of violence.
The army has said it will use force if necessary from Tuesday to restore calm.
More than 60 people have been killed in Lebanon since the fighting started last week in the capital, Beirut.
The sectarian violence is the worst since the end of the 15-year civil war in 1990.
An Arab League delegation trying to mediate an end to the fighting is expected to arrive in Lebanon on Wednesday.
The group will include the regional grouping's Secretary General, Amr Moussa, and Qatari Prime Minister Sheikh Hamad Bin Jabr Al Thani.
"What is going on in Lebanon is unacceptable," Mr Moussa said. "We might succeed and we might not, but we have to try."
The BBC's Jim Muir says people in Lebanon are gripped by fear.
Some 180 people have already fled by boat to Cyprus, Cypriot officials told the Associated Press, as many did during the 2006 conflict between Hezbollah and Israel.
At least six people were wounded in Tripoli as Sunni fighters loyal to the government based in the Bab Tebbaneh district exchanged fire with Shia opposition supporters in nearby Jebel Mohsen, officials said.
The army, which had pulled back its troops from the city centre after a lull in fighting on Sunday, is appealing to militants to get off the streets.
The military said it would use force - if necessary - to restore order, starting from 0600 (0300GMT) on Tuesday.
"Following the events of recent days, namely in Beirut and in the mountains, army units have bolstered their deployment in zones of tension and are working to ensure security, re-establish order and ban all armed presence," an army statement said.
"Army units will ban collective or individual irregularities in line with legal procedures, even if this means using force," it said.
Earlier, troops were deployed in the Chouf mountains after an overnight offensive by Hezbollah on the forces of the pro-government Druze leader, Walid Jumblatt, left at least 13 people dead.
Our correspondent says the skirmishes in the Druze stronghold were amongst the most bitter of the past five days and involved heavy weaponry for the first time.
Security officials have told the Reuters news agency that at least 36 people, including 14 Hezbollah fighters, were killed during Sunday.
Eleven people were killed in the town of Chouweifat, officials said.
The fighting was brought to an end on Sunday by the decision of Mr Jumblatt to call a truce and hand control of the whole area over to the Lebanese army, which is now deploying troops there.
But reports from the area said that columns of Hezbollah fighters attacked late at night from the eastern Bekaa valley.
There were also reports of shooting incidents overnight in West Beirut, but the capital is said to be quiet following a similar arrangement with Hezbollah, which withdrew after crushing supporters of the government.
The latest violence erupted after the government moved to shut down Hezbollah's telecoms network and remove the chief of security at Beirut's airport for allegedly sympathising with Hezbollah.
Arab League foreign ministers meeting in Cairo have urged an immediate halt to the fighting in Lebanon and agreed on Monday to send a ministerial delegation to Beirut to try to mediate an end.
According to reports in some Lebanese papers, one proposal being discussed is the idea that the current Western-backed government should resign and had over to a military council.
The army has emerged as the only factor preventing a complete collapse, and it is generally agreed that its commander, Gen Michel Suleiman, should be the next president.
For the past 16 months, Lebanon has been locked in political stalemate between the ruling coalition and Hezbollah-led opposition over the make-up of the government.
It has also not had a president since November, when Emile Lahoud stepped down despite parliament failing to agree on his successor.
Lebanon was plunged into civil war from 1975-90, drawing in Syria and Israel, the two regional powers.