The bombing took place near the main government compound in the predominantly Shiite region.
Ambulances were seen rushing to the area to evacuate the injured.
No group has claimed responsibility for the attack.
Hermel is in Lebanon's Bekaa Valley near its northern border with Syria and is known as a stronghold of the Shi'ite Hezbollah.
Local channels, including Hezbollah's Al Manar, broadcast images of a large plume of smoke pouring up from a square and the twisted wreckage of a car.