At least 14 people died and 120 were wounded in a south Beirut Shiite Hezbollah neighbourhood after the explosion of a powerful car bomb that also trapped other people in burning buildings.
The blast, which sent a cloud of black smoke over the Lebanese capital, damaged at least five buildings in the Ruwais district, which is one of Hezbollah militant group's strongholds.
Sunni group the Brigades of Aisha claimed responsibility, saying the target was Hezbollah.
"This is the second time that we decide the place of the battle and its timing and you will see more, God willing," said a masked man, flanked by two others brandishing rifles, in a video statement addressed to Hezbollah leader Sayyed Hassan Nasrallah.
The facades of several buildings were damaged and graphic images of charred bodies were seen inside cars caught in the explosion. The scene was close to the Sayyed al-Shuhadaa (Martyrs) complex where Hezbollah leader Nasrallah addresses followers.
It was the second blast in just over a month as groups opposed to Syria's president Bashar al-Assad have promised to take revenge against Hezbollah backing him. Lebanon's Sunni Muslims mostly support the rebels in Syria.
It is not clear whether the explosion targeted any Hezbollah figure. "I don't know what happened. It's as if we were struck by an earthquake," one witness told Reuters.
Nasrallah remained adamant that his group would continue fighting for Assad.