Troops were out in force, setting up sandbags and checkpoints along many roads.
Seven people were killed in Sunday's riots and at least 40 were injured, including activists from the Syrian-backed opposition parties Amal and Hezbollah, a security official told AFP.
Local newspapers and television reported that the violence broke out after youths protesting power cuts in the Shia district of Shiyah entered the nearby Christian area of Ein El Rommaneh and began throwing stones and setting cars on fire. A grenade was also thrown injuring seven people.
The situation quickly escalated with youths moving in several neighbourhoods, setting tyres ablaze and briefly shutting down the main road leading to the airport. Protests also broke out in the southern coastal cities of Sidon and Tyre and in the eastern Bekaa region.
The bloodshed raised fears of civil unrest in a country already grappling with its worst political crisis since the end of the civil war and with a series of assassinations mainly targeting anti-Syrian figures.
The latest attack on Friday involved a massive car bomb that killed a top intelligence officer.
"We call on the army to clearly explain which side is behind the killing of these innocent people," said the group, backed by Syria and Iran.
"Otherwise another party fired the shots and we want to know who."
Newspapers carried ominous headlines warning of civil war if the situation got out of hand.
"The demons of discord are attempting to reignite the fires of the civil war," screamed a headline in the French daily L'Orient-Le-Jour, which is close to the ruling coalition.
Al Diyar newspaper, which is close to the opposition, said Sunday's clashes harked back to the dark days of the civil war as the protests began in the Mar Mikhael area of southern Beirut, near the site of the massacre of Palestinians that triggered the war.
Prime Minister Fuad Siniora declared a day of mourning Monday and ordered all schools and universities shut.
"This is an hour of sadness. Our country is passing through the most dangerous times," he said.
The ministers urged Lebanese lawmakers to elect army chief Michel Sleiman at a parliament session scheduled for February 11 after numerous delays. (AFP)
Lebanon on high alert after deadly riots