A suicide bomber has killed at least two people in an attack on the Prophet's Mosque in Medina, the second most holy site in Islam.
State media reports that two members of the Saudi security forces were killed in the explosion near the mosque's security headquarters.
Pictures on social media show dark smoke billowing from flames near the mosque, where the Prophet Mohamed is buried.
The blast is believed to have occurred at sunset, when people were beginning to break their fast for Ramadan.
Qari Ziyaad Patel, 36, from Johannesburg, South Africa, was in the mosque when he heard a loud blast just as sunset prayers were ending and people were breaking their fast.
He said many initially believed it was the sound of traditional, celebratory cannon fire but then he felt the ground shake.
"The vibrations were very strong...it sounded like a building imploded", he said.
Muslims believe Prophet Mohamed built the mosque himself shortly after arriving in Medina after fleeing Mecca.
Around two million pilgrims have already visited the mosque on their way to Mecca during Ramadan, al-Arabiya reports.
The attack was the third suicide attack in less than 24 hours
A suicide bomber blew himself up in the car park of a mosque in the eastern city of Qatif – where most of the country's Shia minority live.
Another suicide attacker struck in Jeddah, on the west coast, in the early hours of Monday morning after detonating a device near the US consulate.
Two security officers were injured as they tackled the man to the ground.
The attacks come on the eve of Eid al-Fitr which marks the end of the holy month of Ramadan.
The possibility of coordinated, multiple attacks across different cities in Saudi Arabia on the same day underscores the threat the kingdom faces from extremists who view the Western-allied Saudi monarchy as heretics and enemies of Islam.
Saudi Arabia is part of the US-led coalition fighting Isis in Iraq and Syria.
Additional reporting by agencies
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