Witnesses said Monday that they saw body parts at the site of the explosion near Qatif's Faraj al-Omran mosque, which is frequented by people in the holy month of Ramadan.
The first explosion in Qatif destroyed a car parked near the mosque, followed by another explosion just before 7 p.m. local time.
Local media said an attacker detonated his bomb when he was attempting to enter the mosque.
Nasima al-Sada, a resident of Qatif, told AFP that "one bomber blew himself up near the mosque," which is usually used by Shias in downtown Qatif. She added that no bystanders were hurt in the blast.