"Suicide bomber for sure. I can see the body" torn apart, said a witness to the attack in the Shiite-populated city of Qatif.
Nasima al-Sada, another resident, told AFP that "one bomber blew himself up near the mosque", frequented by Shiites in downtown Qatif, on the Gulf coast.
No bystanders were hurt, she said.
The blast, near the end of the Muslim holy fasting month of Ramadan, was the second suicide bombing today in the predominantly Sunni kingdom.
