Two Shiites were killed in overnight clashes with Saudi police in the eastern province of Qatif following the arrest of a prominent Shiite cleric and government critic.
Akhbar Shakuri and Mohammed Filfel died and a dozen other protesters were wounded during clashes that erupted when police opened fire to disperse a demonstration against the arrest of Sheikh Nimr Al Nimr, activists said.
The violence occurred in Riyadh Street, the main artery of Qatif City, they said. The reports could not be independently verified.
The interior ministry described Mr Nimr as an "instigator of sedition" as it announced that he was arrested at Al Awamiya in Eastern Province yesterday, after being wounded in the leg while putting up resistance.
He was transferred to hospital and was due to be interrogated, ministry spokesman Mansur Turki said, cited by the official SPA news agency.
Mr Nimr is considered one of the main advocates of demonstrations that first took place in February 2011 after an outbreak of violence between Shiite pilgrims and police in the holy city of Medina.