Two policemen and two suspected gunmen have been killed in the Saudi town of Buraida, north of the capital Riyadh, in a security operation linked to an earlier shooting in the east of the country.

Six people were also arrested on Tuesday in several other raids across the country, hours after five people were killed and nine others injured late on Monday in the Eastern Province.

An interior ministry spokesman said security forces raided several places in the cities of Riyadh, al-Ahsa, al-Khobar and throughout the Eastern Province, and apprehended those suspected of being involved in the shooting.

Referring to Monday's attack in the town of al-Dawla in the city of al-Ahsa, Major General Mansour Al-Turki said: "As a group of citizens was leaving a building ... three masked men opened fire at them with machine guns and pistols."

The official did not provide further details. Activists on social media said the shooting targeted Shia Muslim worshippers as they were leaving a shrine.

The attack came coincided with the annual commemoration of Ashoura, the most revered religious day in the Shia calendar.

Footage posted online showed corpses lying in pools of blood after the attack in the oil-rich eastern region, where most of Saudi Arabia's two million Shias live.

Bloodstains were seen purportedly on the carpet of the hall where the commemorations were being held.

The Ashoura commemorations mark the killing of Imam Hussein, the grandson of the Prophet Mohammed, by the army of the Caliph Yazid in 680 AD - an event that lies at the heart of Islam's sectarian divide into Shia and Sunni sects.

Saudi Arabia's supreme council of Sunni clerics condemned Monday's attack as "criminal," urging Saudis to "close ranks in standing up against the treacherous criminals".

"The enemies of our religion and our homeland aim to attack our unity and stability," the council said in a statement.

Protests and sporadic attacks on security forces have wracked Shia areas of the Eastern Province, where the minority community complains of marginalisation.

Tensions escalated last month after a Saudi court handed down a death sentence against leading Shia cleric Nimr al-Nimr, a driving force behind the demonstrations.

Source: Al Jazeera and agencies