A Red Cross official said 37 people had been hurt in the blast, in the Beir el-Abed area.
TV pictures showed vehicles on fire and a cloud of black smoke. No group has said it was behind the attack.
Rebels in neighboring Syria have threatened to target Hezbollah, which intervened in the fighting there to support President Bashar al-Assad.
Two rockets hit south Beirut in May.
The explosion was caused by a car bomb in the car park of an Islamic centre, Hezbollah-run al-Manar TV said.
Dramatic footage broadcast by the station showed firefighters trying to put out the flames.
“This is a message, but we will not bow,” Hezbollah official Ziad Waked told al-Manar, AP reports.
Hezbollah fighters were instrumental in Syrian government forces’ strategic victory in Qusair in early June.
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