Heavily armed Shia gunmen attacked Druze areas such as Choueifat
Control of several villages loyal to Lebanon's pro-government Druze leader Walid Jumblatt has been handed to the army after an attack by Hezbollah.
The group's fighters used heavy weapons and small arms to attack the mountain settlements south-east of Beirut.
A truce was called after the Druze capitulated to avoid bloodshed, a BBC correspondent reports.
It follows four days of fighting in which Hezbollah stormed west Beirut, raising fears of a return to civil war.
About 40 people have died in total in the clashes, which pitch the Syrian-backed Shia Islamist movement Hezbollah and its allies against the governing Western-backed Sunni, Christian and Druze alliance.
Beirut was quiet on Sunday, after control of areas seized by Hezbollah was handed to the Lebanese army, but clashes took place overnight in Lebanon's second city, Tripoli.
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.
Lebanon's problems are part of a much bigger struggle, which is why they are so dangerous for the Middle East, says BBC Middle East Editor Jeremy Bowen, who is in the country.
The BBC's Jim Muir in Beirut reports that Sunday's battle began in earnest after some skirmishing and provocations, with a string of Druze villages caught in a barrage of fire.
The BBC's Jeremy Bowen runs for cover as shots are fired at him and his crew
Mr Jumblatt knew Hezbollah, by far the strongest power in the land, could easily storm his entire mountain enclave, so he asked a Druze rival allied to Hezbollah to broker a deal to hand the whole area over to the Lebanese army, he adds.
"I tell my supporters that civil peace, coexistence and stopping war and destruction are more important than any other consideration," Mr Jumblatt told the Lebanese Broadcasting Corporation.
A ceasefire was arranged, and it seems to be generally holding, our correspondent says.
The army is also filling the vacuum in west Beirut since Hezbollah's withdrawal on Saturday.
Violence erupted in Beirut after the government moved to shut down Hezbollah's telecoms network and remove the chief of security at Beirut airport for alleged sympathies with Hezbollah.
The group's leader, Hassan Nasrallah, called the move a "declaration of war".
Seen by many as neutral, the army has emerged as an arbiter
But the army, which correspondents say remains trusted by most of Lebanon's competing political factions, overturned both measures after Hezbollah gunmen seized control of swathes of the city.
On Sunday many roads in the capital remained blocked, including the airport road, as the Shia group continued a campaign of civil disobedience.
In Tripoli, Sunni supporters of the government have reportedly been fighting members of an Alawite sect loyal to Hezbollah with machine-guns and rocket-propelled grenades.
At least one person was killed over the weekend and thousands are believed to have fled their homes.
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 country has been without a president since late 2007, although there is general consensus that the head of the army, Gen Michel Suleiman, would make the best compromise candidate.
The Arab League delegation agreed on in Cairo will be led by the 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.
The delegation's mediation mission will not be easy, the BBC's Heba Saleh reports.
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.
The fighting in Lebanon is seen as a disaster by pro-Western Arab countries such as Egypt and Saudi Arabia, which are concerned about the influence of Hezbollah's other ally Iran, our correspondent adds.
Lebanon was plunged into civil war from 1975-90, drawing in Syria and Israel, the two regional powers.
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