The UN human rights chief has called a suicide bombing outside the Prophet Mohammad's Mosque in the Saudi city of Medina an attack on Islam itself, and many Muslims are expressing shock that their second-holiest site has been targeted.
Three apparently coordinated suicide attacks on Monday targeted Medina, the US consulate in Jeddah and the largely Shi'ite Muslim city of Qatif.
At least four security officers were killed.
No group has claimed responsibility but Islamic State (IS) has carried out similar bombings in the US-allied kingdom in the past year, targeting Shi'ites and Saudi security forces.
Zeid Ra'ad al-Hussein, the UN High Commissioner for Human Rights and a member of the Jordanian royal family, delivered his remarks via a spokesman in Geneva on Tuesday.
"This is one of the holiest sites in Islam, and for such an attack to take place there, during Ramadan, can be considered a direct attack on Muslims all across the world," he said, referring to the Islamic holy month.
"It is an attack on the religion itself."
The militant attacks on Medina are unprecedented. It is home to a mosque built by the Prophet Mohammed, the founder of Islam, and houses his tomb.
Attacks on Mecca, the most sacred place in Islam, are also extremely rare. The Al Saud ruling family considers itself the protectors of both sites. IS says the Saudi rulers are apostates and has declared its intention to topple them.
Iran condemned the attacks.
Saudi Arabia's crown prince and anti-terror tsar, Mohammed bin Nayef bin Abdul-Aziz, sought on Tuesday to reassure Saudis of the country's security.
"The security of the homeland is good, it is at its highest levels and thanks be to God it gets stronger every day," the state news agency SPA quoted him as saying.
Prince Mohammed has been credited for successfully ending a bombing campaign by al-Qaeda in Saudi Arabia between 2003-2006.
Monday's bombings came days before the end of the holy month of Ramadan when Muslims fast from dawn until dusk.
Saudi security officials say the IS's supporters inside the kingdom mainly act independently from the group in Iraq and Syria.
Salah al-Budair, the imam of the Prophet's Mosque, warned young people about being lured by the "malignant" ideology of Islamic State.