A powerful car bombing Thursday killed at least 14 people and wounded about 200 others in a Beirut stronghold of pro-Damascus Shiite group Hezbollah, national news agency NNA said.
A previously unknown group believed to be a Syrian rebel cell said it carried out the attack.
The bombing, reminiscent of the frequent attacks during Lebanon's 1975-1990 civil war, caused enormous damage to buildings.
It set cars ablaze in Beirut's southern suburbs, a bastion of the militant group which backs Syria's embattled President Bashar al-Assad.
A military source earlier told AFP on condition of anonymity that the explosion, targeting an area between the Bir al-Abed and Rweiss neighbourhoods, was caused by a car bomb.
It sent a thick plume of black smoke into the Mediterranean sky.
"Preliminary reports say the bodies of 10 people ... were taken to Sahel hospital, as well as 42 wounded, while four bodies... and 100 wounded were taken to Rasul al-Aazam hospital," NNA said.
The agency, quoting security sources, said another 50 wounded were taken to other hospitals.
Hezbollah's television channel showed firemen helping residents trapped in their homes escape the flames, as well as a crowd of people in panic and rage, gathered near the site of the explosion.
"Terrorism has struck the southern suburbs again," said Al-Manar's television presenter, adding that Hezbollah was "paying the price for its position".
Just over a month ago, the Bir al-Abed area was hit by a car bomb attack later claimed by another little-known Syrian rebel group. Dozens of people were injured in that attack.
Thursday's explosion came a day after Nasrallah said in an interview that his group was taking steps to ensure July's attack was not repeated.
Shortly after news of the latest attack broke, an online video surfaced showing three masked men, two of them holding rifles, in front of a white flag inscribed with the Islamic profession of faith.
"We... send a message to (Hezbollah chief) Hassan Nasrallah's pigs," said one of the men, wearing a white mask.
Hezbollah, whose military wing was blacklisted by the European Union in July, has been fighting in Syria alongside Assad's troops to help crush an anti-regime rebellion.
The Bir al-Abed car bomb attack, which wounded 53 people, was the most serious incident in Hezbollah's Beirut stronghold since the March 2011 outbreak of the conflict in neighbouring Syria.
The mainstream rebel Free Syrian Army condemned the attack.