Hezbollah forces quickly cordoned off the area and prevented journalists from entering the town of Nabi Sheet, 30 km (18 miles) south of the regional center of Baalbek.
The cause of the explosion was not immediately clear. One source said the blast occurred in a weapons cache but a security source said the building was used to store car parts.
Despite domestic demands for the group to disarm, Hezbollah keeps stockpiles of weapons - including thousands of rockets, saying they will need them in the event of a new war with Israel, a country it fought a month-long war in 2006.
Baalbek is known as a hub for arms and drug smuggling.