The Lebanese army remains on high alert a day after bloody clashes involving troops and Shia demonstrators.
The army has set up set up checkpoints along many roads
Seven people died as troops tried to remove barricades of burning tyres during a protest over power cuts in the southern suburbs of Beirut.
The Western-backed government has declared Monday a day of mourning, with schools and universities shut.
Lebanon has had no president amid disputes between the government and the Syria-backed opposition since November.
In scenes echoing Lebanon's 15-year civil war, troops set up sandbags and checkpoints along many roads near mainly Shia neighbourhoods of the capital on Monday.
There were fears the unrest might spread to nearby Sunni Muslim and Christian areas.
Cars were torched as protests spread around Beirut
The BBC's Jim Muir in Beirut says the involvement of the army is significant, because it is generally regarded as politically neutral.
As the military investigated the killings, the Shia militant group Hezbollah demanded to know who was responsible.
The group said in a statement: "Did those who fell as martyrs and were wounded fall by the army's bullets, and if so, who issued the order for the soldiers to fire?"
The demonstration in Beirut's Mar Makhaeil suburb had been over chronic electricity cuts in mainly Shia areas of the capital.
Violence spread after the death of an official from Shia opposition faction Amal.
The movement and ally Hezbollah are locked in a power struggle with the government of prime minister Fouad Siniora.
In the nearby suburb of Ain Roummaneh, the site of a massacre that triggered Lebanon's 1975-1990 civil war, a hand grenade wounded seven people.
The violence was worst since the country plunged into a crisis three years ago with the assassination of former PM Rafik Hariri.
His death sparked international outrage and forced the Syrian army to withdraw from its neighbour after 30 years of control.
Also on Sunday, foreign ministers from the League of Arab States in Cairo called for Lebanon to break its political deadlock by electing army chief Michel Suleiman as president.
A massive bomb blast on Friday killed a senior Lebanese intelligence officer and three others in the east of the capital.