Hezbollah's Al-Manar television confirmed the explosion and said an unspecified number of people were wounded in the attack near the Shiite village of Sbuba.
Lebanon has been deeply divided over the war in neighbouring Syria, and has seen rising unrest since Hezbollah announced it had dispatched fighters to aid President Bashar al-Assad's regime in its war on Sunni-led rebels.
Al-Manar said an SUV parked near the Hezbollah post blew up when members of the militia, which controls much of the Bekaa Valley in eastern Lebanon along the border with Syria, approached the vehicle.
"Other Hezbollah members rushed to the scene and opened fire, thinking the others had been ambushed, and then helped the wounded," it said.
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An AFP photographer saw four burned out vehicles at the post, one of many manned by Hezbollah in the Bekaa, which hosts tens of thousands of Syrian refugees.
Residents told AFP the position was used by the militia as a staging post for its vehicles and fighters, especially those headed to Syria to fight alongside government forces.
Lebanon's NNA news agency reported that militiamen of Iranian-backed Hezbollah had already intercepted two car bombs earlier on the same road.
Later on Wednesday, six rockets fired from Syria rained down on the Bekaa town of Hermel, with one hitting an army barracks, wounding two soldiers, according to a military statement.
A security source had earlier said four people were wounded by rockets that struck residential areas in Hermel, another Hezbollah stronghold which has been hit by cross-border rocket fire several times in the past.