A suicide bombing and gun attack on Shi'ite Muslim worshippers killed at least four people in eastern Saudi Arabia on Friday, the interior ministry and witnesses said, extending a spate of attacks on the kingdom's Shi'ite minority.
The assault on the Imam Rida mosque in the Eastern Province town of Mahasen, a mixed Sunni-Shi'ite district in which there is an extension of a compound where state oil company Aramco employees live, also wounded at least 18 people.
There was no early claim of responsibility but it resembled previous attacks by Sunni militants from Islamic State on Shi'ites it considers to be heretics. The oil-producing Eastern Province is home to Saudi Arabia's Shi'ite community.
It came less than a month after Sunni-ruled Saudi Arabia executed 47 people, most of them al Qaeda militants convicted of attacks in the world's biggest oil exporting state since 2003, as well as dissident Shi'ite cleric Nimr al-Nimr.
The Saudi interior ministry said security forces prevented two suicide bombers from entering the mosque, where one blew himself up, killing four people. Security forces exchanged fire with the second man and arrested him.
Witnesses said one suicide bomber blew himself up outside the mosque, causing a power blackout inside. They said worshippers overpowered a second attacker after he opened fire in the mosque where 200 people were performing Friday prayers.
"The explosion happened outside the mosque, at the courtyard of the mosque, while another one entered with a machinegun. There are martyrs and wounded," one witness said in an audio message circulated on social media.
"The young men grabbed his machine gun and beat him up, but he did not die. The police then came and took him away and the wounded were taken in private cars because ambulance cars did not arrive quickly."
Another witness, speaking to Reuters by telephone, said a third attacker was believed to be involved in the attack and that he may have fled or disappeared.
A video recording provided by activists showed a crowd surrounding a man prone on the floor, turning him over and unfastening what they said was a suicide belt around his waist.
Witnesses had earlier said three people died in the attack.
Saudi Arabia has suffered a string of deadly shooting and bomb attacks in recent months, many of them claimed by the ultra-radical Islamic State.
Islamic State is bitterly hostile to Gulf Arab monarchies and is seen to be trying to stoke Sunni-Shi'ite sectarian confrontation within Arabian peninsula states to destabilize and ultimately overthrow their dynasties.
Al Qaeda has warned Saudi Arabia it will pay for the executions of dozens of its members on Jan. 2, saying they were intended to be a new. year's gift to Riyadh's Western allies aimed at consolidating the Saud dynasty.