Rights groups accused the police of beating protesters during previous rallies in Qatif.

An interior ministry spokesman told reporters that police had fired over the heads of protesters on Thursday.

The spokesman added that three people, including a policeman, had been injured.

The unrest comes amid calls over the internet for a so-called "day of rage" protest in cities throughout the country after Friday prayers.

Analysts say it is unclear whether anyone will heed the calls, as Saudi Arabia has so far not seen protests on the same scale as other nations in the Middle East and North Africa.

Shias, who are mainly concentrated in the east of the country, make up about 10% of the population in Sunni-dominated Saudi Arabia.

The region borders Bahrain, a Shia-majority kingdom ruled by a Sunni government that has been rocked by anti-government protests since mid-February.

Amid signs of growing unrest in the region, Saudi Arabia's King Abdullah recently unveiled $37bn (£22.7bn) in benefits for citizens, including a 15% pay rise for state employees.