AITA AL SHAAB, Lebanon — Thousands of men, women and children gathered in this village near the border with Israel, jumped to their feet, pumped their fists and cheered as Hassan Nasrallah, the leader of Hezbollah, vowed on Friday to step up the fight against the radical Sunni Muslims whom he accused of a car bombing on Thursday in one of the group’s strongholds in Beirut.

The death toll from the blast in Beirut’s southern suburbs rose to 24 on Friday, making it the deadliest attack in Lebanon in decades. Many Lebanese saw the attack as payback for Hezbollah’s military support for the government of President Bashar al-Assad in the civil war in Syria.

Addressing the attackers, Mr. Nasrallah insisted the bombing had not affected the group’s position. “If you think that by killing our women, by killing our children, by killing our innocents,” enemies will make Hezbollah stop aiding the Syrian government, “you are wrong,” he said.

In fact, he said, such attacks would lead Hezbollah to double the size of its forces in Syria, where, he said, they were fighting takfiris, or extremists, who consider all but those who follow their school of thought heretics.