Heavy fighting has broken out again in the northern Lebanese city of Tripoli between opposition gunmen and government supporters, officials say.
Overnight, at least 13 people were killed as Hezbollah attacked forces of the pro-government Druze leader, Walid Jumblatt, in the mountains near Beirut.
The fighting in the Chouf mountains later died down after an agreement that the army would be deployed in the area.
The Arab League is to send a delegation in an attempt to end the conflict.
Its Secretary-General, Amr Moussa, said it wanted to save Lebanon.
"What is going on in Lebanon is unacceptable. We are very worried about this," he said. "We might succeed and we might not, but we have to try."
A date for the mission will be set after talks with the various Lebanese factions.
The sectarian violence, which broke out when Hezbollah gunmen fought pro-government forces in Beirut last week, is the worst since the end of the long-running civil war in 1990.
At least 47 people have been killed.
The BBC's Jim Muir says people are gripped by fear and wondering where the crisis goes from here.
Until the armed clashes resumed in Tripoli on Monday afternoon, Lebanon had been largely quiet following a weekend of fighting in the northern port city and the Chouf mountains.
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 told the Associated Press.
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, on the other side of the mountains.
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.
Activity and movement in the city are still at very low levels, our correspondent adds. Some main routes are still blocked, the international airport remains closed and getting in an out of Lebanon remains very difficult.
The fighting has pitched the Syrian-backed Shia Islamist movement Hezbollah and its allies against the pro-Western governing Sunni, Christian and Druze alliance.
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 foreign ministers meeting in Cairo have urged an immediate halt to the fighting in Lebanon and agreed to send a ministerial delegation to Beirut to try to mediate an end to the crisis.
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.
It has been reported also that the American guided missile destroyer the USS Cole has crossed back into the eastern Mediterranean through the Suez Canal.
Our correspondent says that prompted a warning from sources in the Hezbollah-led opposition that any hint of US intervention would lead the opposition to abandon the few red lines they have been observing in their campaign to undermine the government.
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.
The Arab League delegation agreed on in Cairo will be led by Qatari Foreign Minister Sheikh Hamad bin Jassem.
Ministers also decided to give what they describe as logistical support to the Lebanese army to help it maintain security.
An existing Arab League initiative aimed at facilitating the election of a Lebanese president has been deadlocked for months and Syria, a key Hezbollah ally, stayed away from Sunday's meeting.
Lebanon was plunged into civil war from 1975-90, drawing in Syria and Israel, the two regional powers.
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