A suicide bomber detonated an explosives vest during Friday weekly prayers at a Shiite mosque in eastern Saudi Arabia, killing and wounding several people, officials said.
The kingdom's Eastern Province has been hit by previous attempts by Sunni extremists to foment sectarian tensions with its large Shiite community, including a deadly shooting in November.
The interior ministry said a suicide bomber detonated a bomb at the mosque in Shiite-majority Qatif, SPA news agency reported.
"An individual detonated a bomb he was wearing under his clothes during Friday prayers at Ali Ibn Abi Taleb mosque in Kudeih in Qatif province," the ministry spokesman said in a statement.
He did not give a specific casualty toll but said several people were killed and wounded in the attack.
An activist said at least four worshippers were killed and others wounded, and news websites in eastern Saudi Arabia posted photographs of bodies lying in pools of blood.
Qatif hospital issued an urgent call for blood donations after the attack and called in off-duty staff to cope with the high number of casualties, the activist said.
Naseema Assada, a resident of Qatif, said worshippers were celebrating the birth of revered Shiite Imam Hussein when the blast occurred.
"The people are very angry," she said, adding that they tried to stop police from entering the Kudeih area.
Residents had feared such an attack was coming, she said, because the government was failing to curb hate speech on social media against the Shiite community.
"We don't want a repeat of what is happening in Syria or Iraq here. This is our country and we love it."
The mufti of Saudi Arabia, the highest-ranking Sunni cleric, denounced the attack in a statement broadcast on state television.
"It is a criminal act aimed at dividing the sons of the nation... and at sowing trouble in our country," he said.
The interior ministry spokesman said Saudi Arabia would "hunt down anyone involved in this terrorist crime carried out by people seeking to undermine national unity."
The website of Arryadh newspaper posted pictures showing bloodied prayer rugs and part of the ceiling of the mosque that had caved in.
First reports by witnesses said the suicide bomber appeared to be from Pakistan but others said he was wearing traditional Afghan clothes.
Kudeih is in the oil-rich Eastern Province, home to most of Saudi Arabia's minority Shiite community which has long complained of marginalisation in the Sunni-ruled kingdom.
Saudi police have made a string of arrests in recent months of Sunni extremists suspected of plotting attacks aimed at stirring sectarian unrest in the Eastern Province.
Last November, gunmen killed seven Shiites, including children, in the eastern town of Al-Dalwa during the commemoration of Ashura, one of the holiest occasions of their faith.
Four men carried out the attack after killing a man from a neighbouring village and stealing his car to use in the shootings, the interior ministry said.
In April, authorities said they arrested 93 jihadists, including 62 suspected of links to the Islamic State group who were plotting attacks to "incite sectarian sedition".
Since 2011, protests and sporadic attacks on security forces have taken place in Shiite areas.