"According to an initial toll, five people have been killed and more than 20 injured," the source said on condition of anonymity.
Hezbollah's Al-Manar television said: "The terrorist explosion targeted a densely populated residential area, just 150 to 200 metres (yards) away from Hezbollah's political bureau."
The official National News Agency reported that the explosion was caused by an explosives-packed four-wheel-drive vehicle in the Haret Hreik neighbourhood.
The district is a symbolic one for Hezbollah, which once based many of its leadership institutions in the area.
Massive Israeli air bombing that accompanied its 2006 war with Hezbollah, but it has since been rebuilt.