At least 21 people killed after suicide bomber detonates explosives during Friday prayers in Qatif province.
A Saudi policeman has been killed and two others wounded in an attack in a mainly Shia area of Eastern Province, the interior ministry has said.
“One policeman was killed and two injured,” a spokesman said on Wednesday.
In a statement, police said two suspects were arrested after a patrol came under fire in al-Jesh village in Qatif district late on Tuesday.
It was not immediately clear who was responsible for the attack.
Most of Saudi Arabia’s Shias live in the oil-rich east, where many say they face marginalisation.
Eastern Province was the scene of periodic clashes involving security forces after demonstrations broke out four years ago alongside a Shia-led protest movement in neighbouring Bahrain.
Since late last year, the Saudi region has been targeted by the Islamic State of Iraq and the Levant group, which consider Shias to be heretics.
Authorities, and analysts, say the armed group have tried to ignite sectarian tensions in the Sunni-majority kingdom but have failed.
On successive Fridays in May, suicide bombings at Shia mosques in Eastern Province, one of them in Qatif, killed a total of 25 people.
An ISIL-affiliated group calling itself Najd Province, which takes its name from the region around Riyadh, claimed those attacks, as well as another suicide bombing that killed 26 people at a Shia mosque in Kuwait last month.
Saudi security forces have also been targets of attacks linked to ISIL, which has committed atrocities in Iraq and Syria and inspired attacks elsewhere around the world.
On July 18, the interior ministry announced it had disrupted a network linked to ISIL and made more than 430 arrests, foiling new attacks on Shia mosques and a diplomatic mission.