Saudi Arabian security forces have fired on demonstrators in the city of Qatif on the western shore of the Gulf, a report by US broadcaster CNN said.
Two witnesses and an activist told the channel that three injured people were taken to hospital during Thursday’s demonstration. The CNN report said it was unclear whether live ammunition or rubber bullets were used.
More than 100 demonstrators in the mainly Shiite-populated city had demanded that the authorities free Shiite prisoners.
A Day of Rage -- modelled on similar protests in other parts of the Arab world -- was planned for Friday in Saudi Arabia. Demonstrations are strictly forbidden in Saudi Arabia. Shiites are a minority in the country, which is under the absolute rule of a Sunni royal family.
The city of Qatif had previously seen demonstrations by Shiites complaining of discrimination and demanding the release of prisoners.