A protester holds up a picture of Sheikh Nimr al-Nimr during a rally at the coastal town of Qatif, against Sheikh Nimr's arrest
Coastal town of Qatif witnessed Shi'ite demonstrations last year against the arrest of Sheikh Nimr al-Nimr (Reuters)

An alleged suicide bomber has detonated explosives at a Shi'ite mosque in the eastern Saudi Arabian district of Qatif during Friday prayers, which has left several casualties.

One witness told Reuters that a huge explosion took place at the Imam Ali mosque in the village of al-Qadeeh.

He estimated there were at least 30 casualties in the attack, where more than 150 people were praying. Graphic pictures shared on social media showed victims covered in blood being taken away on stretchers. Other pictures showed shattered glass, debris next to the tiled pillars and bodies covered in sheets laid out on the floor.

The Saudi state news agency confirmed that a blast had occurred at a mosque in eastern Saudi Arabia. A spokesman for the Saudi Interior Ministry said the bomber detonated a suicide belt hidden under his clothes inside the mosque causing several people to be "martyred or wounded".

"Security authorities will spare no effort in the pursuit of all those involved in this terrorist crime," the official said in a statement carried by state news agency SPA.

In March 2011, Qatif was the epicentre of Shi'ite demonstrations against what they perceived as decades of discrimination and religious and political repression from the Sunni kingdom.
It was the beginning of an uprising that was met with a crackdown, a wave of arrests and cases of police allegedly firing on unarmed protesters.

Since then, 27 have been killed and more than 250 imprisoned, according to local Shia leaders and human rights activists in the towns and cities of eastern Saudi Arabia, who fear rising sectarianism fuelled by Riyadh's war against Shia Houthi rebels in Yemen.

There are 2.7 million Shia in Saudi Arabia, making up 12% of the population, with most living in al-Ahsa and al-Qatif districts in the country's eastern province, which also contains the bulk of the kingdom's oil.

Ahmed al-Omran, Saudi-based Wall Street Journal correspondent, said that Qatif hospitals are seeking blood donors for victims of the attack. The mosque bombing is the second attack on a Shi'ite place of worship in the Gulf kingdom after the Dalwah attack last November, he reported on Twitter.