Hezbollah's Al-Manar TV said the blast was a car bomb inside a parking lot near an Islamic center. The station broadcast footage of a thick plume of smoke rising into the sky at the site of the blast as people rushed to take casualties to the hospital.
Some Syrian rebel groups have threatened to strike in Lebanon after Hezbollah joined Syrian President Bashar al Assad's troops in their battle against opposition fighters.
In May, two rockets slammed into a Hezbollah stronghold in south Beirut, wounding four people. The rockets struck hours after Hezbollah leader Sheikh Hassan Nasrallah vowed in a speech to help propel Assad to victory in Syria's civil war.
Hezbollah has openly joined the fight in Syria, and the group's fighters were instrumental in a recent regime victory when government forces regained control of the strategic town of Qusair near the Lebanese border.