A car-bomb explosion has killed at least 22 people in Beirut's southern suburbs, according to the Lebanese Interior Ministry.
Thursday's explosion struck near a facility used by Hezbollah, the Shia political party and armed group that in recent months has become increasingly and more publicly involved in the conflict in neighbouring Syria.
At least 290 people were injured in the blast, which occurred between the Bir el-Abed and Roueiss neighbourhoods - an area regarded as the heart of Beirut's Hezbollah territory.
The public space is used by Hezbollah to hold neighbourhood residents and supporters whenever Hassan Nasrallah, the group's head, delivers a speech.
The blast put buildings and cars on fire and sent a column of black smoke over the densely populated area.
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," Hezbollah's AlManar's television said, adding that the group was "paying the price for its position".
Najib Mikati, Lebanon's caretaker prime minister, declared Friday a national day of mourning, and called on the Higher Defence Council to hold a meeting.
An unknown Syrian Sunni group called Aisha Umm-al Mouemeneen claimed responsibility for the bombing.
Shortly after the attack, 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.
Al Jazeera's Zeina Khodr, reporting from Beirut, said Hezbollah has been receiving threats from some groups linked to the Syrian opposition.
'We know Hezbollah lost a lot of popularity after it sent its fighters into Syria to support the Syrian regime against the opposition" she said.
Omran al-Zoubi, Syrian minister of information, described the attack as "a coward act in the service of the Israeli enemy".
"Syria strongly condemns that terrorist act and offers condolences to the families of victims and hopes quick recovery for the wounded," Zoubi said in a statement.
Syria conflict spillover
The Free Syrian Army (FSA), the mainstream rebel group fighting against Syrian regime, also condemned the attack.
Syria's conflict has spilled over into Lebanon, where there have been outbreaks of fighting reflecting the sectarian divisions emerging in the Syrian war.
Lebanon's Sunni Muslims mostly support the rebels in Syria, while Shias have largely supported President Bashar al-Assad, who is part of the minority Alawite sect, an offshoot of Shia Islam.
Nasrallah has promised that his group will continue fighting for Assad after it spearheaded the recapture of the strategic town of Qusayr in June.
There have been two attacks in southern Beirut this year - a car bomb on July 9 and two rockets fired into the area two months earlier.