Violence swept the southern suburbs of Beirut on Sunday after demonstrators protesting against power cuts set ablaze tyres, blocking a main road linking the Chiya and Mar Mikhael neighbourhoods.
The army fired warning shots to disperse the demonstrators, a security official said.
Witnesses said armed men in the crowd opened fire at the security forces, who then retaliated.
Security sources revised the death toll from the incident down to six on Monday, down from the previous toll of eight.
Youths wielding sticks and iron bars went on a rampage, pelting cars with stones and setting some on fire.
The army was out in force in a bid to prevent the riots from spreading to nearby Sunni and Christian districts.
Hezbollah, a Shia party leading the opposition against the government, has called for an investigation into the shooting incident.
"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 group said in a statement.
"Or was there another party, and who was it?"
The group said it held the authorities responsible for "every drop of blood spilt" and that any cover-up would be a threat "to stability and civil peace".
General Michel Suleiman, commander of the Lebanese army, pledged on that the army would hold an inquiry into the incident.
The riots were the worst since January 2007 when seven people were killed in clashes between students loyal to rival camps.
An official from the main opposition Shia Amal movement said one of its members, Ahmad Hamza Hamza, 21, was killed.
Officials said more than 19 people were wounded.
The violence escalated after Hamza, who was co-operating with the army, was killed.
It was unclear who fired at the victims, amid reports of snipers shooting into the crowd from rooftops.
Call for calm
Amal and Hezbollah have appealed for calm.
|Sunday's violence came two days after a car |
bomb killed a senior anti-terror official [AFP]
"The situation must be contained. We appeal to all the people who are on the streets to go home so that security forces can restore calm to the region," Ali Hassan Khalil, an Amal MP, said.
He added that his group was not behind the protests.
But the ruling coalition blamed the opposition for the unrest, saying it was being manipulated by Syria and Iran.
"The forces of the Syrian-Iranian axis are fomenting unrest and these events are very dangerous," a statement said.
"The opposition, which answers to Syria and Iran, is solely responsible for the blood spilled today."
The army shut down many roads to stop the protests from spreading, and soldiers also took positions on rooftops.
As night fell, demonstrators temporarily cut the main airport road with burning tyres while gunfire rang out sporadically across the southern suburbs.
A car that had been set ablaze exploded, triggering panic in Beirut where on Friday a massive car bombing killed Captain Wisam Eid, a senior anti-terror officer, and four other people.
A security official warned the riots could spread unless politicians reined in their supporters.
"The politicians alone can decide whether to contain their followers or to give them the green light to spread mayhem," the official said.
"But all indications are that the situation will escalate and that these protests will become our daily fare."
Demonstrators have faced off with security forces on several occasions in the past few days over power cuts and rising prices.