BEIRUT, Lebanon — A blast from a powerful car bomb tore through a southern suburb of this city on Thursday, killing at least 18 people and wounding nearly 300 in the heart of a stronghold of Hezbollah, the militant Shiite movement that has the country’s strongest military force.

The evening blast, which set apartment buildings ablaze and scattered bodies in the street, was the deadliest bombing in Lebanon in more than eight years and raised fears that the country was slipping into a new era of political violence.