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 second-holiest city in Islam after Mecca.
Suicide blasts also struck two other Saudi cities on Monday.
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.
A suspected suicide bomber also died after detonating a device near the US consulate in the city of Jeddah. Two security officers were slightly injured as they tackled the man, but no-one else was hurt.
No-one has yet said they were behind any of the attacks.
They come with the holy month of Ramadan drawing to a close and ahead of the Eid al-Fitr holiday.
Saudi Arabia has been the target of attacks by IS over the past two years.
In June, the interior ministry said there had been 26 "terror attacks" in the kingdom in that time.
A series of deadly attacks worldwide were either claimed by, or blamed on, IS over the past week:
- A suicide gun and bomb attack targeted Istanbul airport on 28 June, killing 45 people.
- Attackers struck a cafe in Bangladesh's capital, Dhaka, last Friday night. Twenty hostages and two policemen were killed.
- A massive truck bomb in Iraq's capital, Baghdad, on Sunday left at least 165 people dead.