The attack came two weeks after Israel killed several Hezbollah fighters and an Iranian general in southern Syria, and four days after Hezbollah struck back by killing two Israeli soldiers in a cross-border attack from Lebanon. On Friday, Hezbollah’s leader, Hassan Nasrallah, made it clear in a speech that the group would not back down in its fight against Israel or in Syria.

The victims in Damascus were not identified as fighters, though at least one appeared in memorial pages on social media in military-looking gear. Syria’s state news agency said that at least one child, a 3-year-old boy, was in critical condition. Extremist Sunni groups among the insurgents, in addition to being sworn enemies of Hezbollah, have targeted Shiites simply for their sect.

Regardless, the attack appeared to be unusually provocative, exploding just outside the ancient fortifications of the Old City, near once-bustling tourist destinations like the Umayyad Mosque that Damascus residents still consider relatively safe and hitting Shiites as they traveled from one revered shrine to another.

The bus’s next destination was to have been the Sayeda Zeinab shrine outside Damascus, particularly revered by Shiites. Protecting that shrine was one of Hezbollah’s early justifications for its intervention in Syria, and the group has used it as a rallying point to recruit fighters.

The bus was part of a weekly tour named for Hussein, the early Islamic warrior whose death in battle is the central narrative of mourning in Shiite Islam, said the organizer, Fadi Kheireddine. He said all 58 passengers were Lebanese.