It was one of the deadliest assaults in recent years in the largest Gulf Arab country, where sectarian tensions have been frayed by nearly two months of Saudi-led air strikes on Shi'ite Houthi rebels in neighbouring Yemen.
More than 150 people were praying when the huge explosion ripped through the Imam Ali mosque in the village of al-Qadeeh, witnesses said.
A video posted online showed a hall filled with smoke and dust, with bloodied people moaning with pain as they lay on the floor littered with concrete and glass. More than 90 people were wounded, the Saudi health minister told state television.
"We were doing the first part of the prayers when we heard the blast," worshipper Kamal Jaafar Hassan said.

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