Since Saturday's execution of four Shiite Muslims in Saudi Arabia, hundreds or thousands of the minority sect have marched nightly in protest, and their anger could herald wider unrest.

The execution of one of them, dissident cleric Nimr al-Nimr, caused an international crisis as Shi'ite Iran and its allies responded angrily, but it also caused upset in his home district of Qatif, where many saw his death as unjustified.

"People are angry. And they are surprised, because there were positive signals in the past months that the executions would not take place. People listen to his speeches and there's no direct proof he was being violent," a Qatif community leader said by phone.

The protests in Qatif, an almost entirely Shi'ite district of about a million people in the oil-producing Eastern Province, have been mostly peaceful, though a fatal shooting and gun attacks on armored security vehicles have also taken place.