A suicide bomber has killed four security officers and injured five others near one of Islam's holiest sites in the Saudi city of Medina, according to the interior ministry.
The bomber detonated his explosives after being stopped outside the Prophet's Mosque, a statement said.
The mosque is the burial place of the Prophet Muhammad and Medina the holiest city in Islam after Mecca.
Suicide blasts also struck two other Saudi cities on Monday.
The fact that an attack happened in Medina at such a place is likely to leave Muslims around the world aghast, BBC World Service's Middle East editor, Alan Johnston, says.
Suspicion is likely to fall on so-called Islamic State (IS), he adds.
Al-Arabiya gave a different account of the incident, saying the bomber had targeted the security officers by pretending he wanted to break his Ramadan fast with them.
Qari Ziyaad Patel, 36, from South Africa, who was in the mosque, told the Associated Press news agency people had at first thought it was the sound of the cannon fire that marks the breaking of fast.
The ground shook, he said, adding: "The vibrations were very strong. It sounded like a building imploded."
Earlier, at least one explosion rocked Qatif, an eastern city which is home to many minority Shia Muslims.
The blast appeared to target a Shia mosque. The attacker was killed but no other casualties were reported.