BEIRUT—A car bomb exploded in the heart of Hezbollah's stronghold in Beirut's southern suburbs on Tuesday, injuring more than 50 people.

The explosion marked the largest attack on the Shiite political and militant group, in an apparent retaliation to Hezbollah's involvement in Syria. No group has yet taken responsibility and there were no fatalities.

Hezbollah has openly fought alongside President Bashar al-Assad's army in Syria in the past few months. In June, Hezbollah led the offensive in the capturing of the rebel-controlled town of Qusayr near the Lebanese border.

Islamist Sunni groups and the Free Syrian Army have issued threats against Hezbollah, warning they would target the group inside Lebanon. Rockets from Syria have slammed Shiite areas in the Bekaa Valley for the past month. In May, two rockets hit a car dealership and an apartment building populated by Shiites near the southern suburbs.

Tuesday's attack fanned fears here that Lebanon was getting more deeply entrenched in Syria's war next door. The country is deeply divided with the majority of Sunnis supporting the opposition while the Shiite back the Assad regime, one of Hezbollah's regional patrons along with Iran.