Flames were seen pouring from the facade of a multi-storey building, with large plumes of smoke.
Local media quoted officials as saying the blast was caused by a suicide bomber.
There has been a spike in sectarian tension in Lebanon blamed on the conflict in neighboring 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.
The blast occurred in Arid Street in the Haret Hreik district. Large crowds gathered at the scene as Hezbollah’s emergency services worked to extinguish the blaze.
Hezbollah sources quoted by the organization’s al-Manar TV said four people had been killed. Several more are reported to have been wounded.
Body parts thought to be those of the suicide bomber were scattered at the scene, Lebanon’s national news agency reported.
Caretaker PM 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 January 2 in the same Beirut district.
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