A gunman was killed during an attack on a police station as four Saudi policemen were wounded in a separate assault on their patrols in the kingdom's Shiite-populated east, state media reported.
"Four masked gunmen on motorbikes entered Al-Awamiya police centre where one of them threw a petrol bomb while the rest opened fire at the station," interior ministry spokesman Mansur al-Turki said, quoted by SPA news agency.
"The guards dealt with them, killing one while the rest (of the assailants) fled," said Turki.
The attack in the Shiite town of Al-Awamiya took place on Friday evening, less than a week after two protesters were killed in clashes with police in Qatif district following the arrest of a prominent Shiite cleric.
In those clashes, on Sunday night, activists said dozens of protesters were wounded when police fired on a demonstration against the arrest of Sheikh Nimr al-Nimr whom the authorities have described as "instigator of sedition."
Turki said the security forces would not tolerate "rioters, especially those who are armed," and would hold responsible anyone who does not help in handing them over or offers them refuge.
In a separate statement on SPA, the spokesman announced that "two security force patrols came under gunfire from masked armed men on motorbikes in the town of Saihat, wounding four members of the security forces."
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He also expressed his "gratitude over the self-restraint and wisdom they had shown as well as their courage in carrying out their duties."
Qatif district, where both towns are located, has witnessed several protests initially triggered in February 2011 after an outbreak of violence between Shiite pilgrims and religious police in the holy city of Medina.
The protests escalated after the kingdom led a force of Gulf troops into neighbouring Bahrain to help crush a month-long Shiite-led uprising last year against that country's Sunni monarchy.
Saudi Arabia's estimated two million Shiites, who frequently complain of marginalisation, live mostly in the east, where the vast majority of the OPEC kingpin's huge oil reserves lie.
Some 37 Shiite clerics released a joint statement on Friday in which they attributed the tensions in the eastern region to the "policy of sectarian discrimination that government and official religious authorities have implemented for decades."
But they urged youths to "steer away from violence" and not to "respond to some who try to provoke you and lure you to violence to mark your movement as terrorist."
They also called on the authorities in the ultra-conservative absolute monarchy to "release all political activists ... including Sheikh Nimr al-Nimr and Sheikh Tawfiq al-Amer."
Amer, who has been detained several times, had called for a "constitutional monarchy" in Saudi Arabia and his arrest prompted activists to push for protests in March 2011.
In May, Amnesty International said seven people had been killed and a number of others injured in clashes between the authorities and protesters in the Shiite-populated region since November.