An attack by a suspected suicide car bomber has reportedly killed four people in a Hezbollah stronghold in a Shia-dominated suburb of Beirut.
Flames were seen pouring from the facade of a multi-storey building, along with large plumes of smoke.
The blast, in Arid Street in the southern Haret Hreik district, left at least 20 injured, reports say.
There has been a spike in sectarian tension in Lebanon blamed on the conflict in neighbouring Syria.
Hezbollah forces have been fighting in support of President Bashar al-Assad in Syria, while Lebanese Sunni Muslims tend to back the Syrian opposition.
Hundreds of people gathered at the scene as Hezbollah's emergency services worked to extinguish the blaze. It is not clear what the target was, though there are Hezbollah offices close by.
A group calling itself the al-Nusra Front in Lebanon put out a statement on Twitter claiming to be behind the attack in revenge for "massacres" perpetrated by Hezbollah.
It is not clear what links the group has to the al-Nusra Front in Syria - an al-Qaeda-linked force fighting Mr Assad's government.
Lebanon's Haret Hreik district is densely populated with many shops. Buildings were damaged and glass strewn over the street.
Details of casualties are sketchy. But of the four deaths reported by Hezbollah's al-Manar TV, quoting Hezbollah officials, one is reported to be a woman.
Body parts thought to be those of the suicide bomber were scattered at the scene, Lebanon's national news agency reported.
Caretaker Prime Minister Najib Mikati called a meeting of the government's emergency response committee.
It is the latest of several recent explosions in Lebanon. Five people were killed and many others injured by a bomb blast on 2 January in the same Beirut district.
Former minister Mohamad Chatah, a Sunni and a critic of Hezbollah, was killed with five others by a car bomb in December.
There are fears that the spiral of violence could tighten further, says the BBC's Jim Muir, who went to the scene.
However, a Hezbollah official and MP told him the organisation did not plan to respond to Tuesday's attack, our correspondent adds.
Lebanon has been politically deadlocked since last March with an alliance led by Sunni former Prime Minister Saad Hariri and Hezbollah unable to agree on a coalition government.