The explosions targeting US diplomats, Shi'ite worshippers and a security headquarters at a mosque in the holy city of Medina follow days of mass killings claimed by the Islamic State group, in Turkey, Bangladesh and Iraq. The attacks all seem to have been timed to coincide with the approach of Eid al-Fitr, the holiday that celebrates the end of the fast.
A Saudi security official said an attacker parked a car near the US consulate in Jeddah before detonating the device. The official said the government was checking the reports of blasts in Qatif and Medina.
In the only one of the three attacks that appeared to have caused many casualties, a suicide bomber detonated a bomb near the security headquarters of the Prophet's Mosque in Medina, the second-holiest site in Islam.
Saudi-owned al-Arabiya television said an initial death toll from the Medina blast included three suicide bombers and two security forces officers.
A video sent to Reuters by a witness to the aftermath of the Medina bombing showed a large blaze among parked cars in the fading evening light, with a sound of sirens in the background.
A picture sent to Reuters showed a burnt and bleeding man lying on a stretcher in a hospital.
Other pictures circulating on social media showed dark smoke billowing from flames near the Mosque of the Prophet, originally built in the 7th century by the Prophet Muhammad, who is buried there along with his first two successors.
In Qatif, an eastern city that is home to many members of the Shi'ite minority, at least one and possibly two explosions struck near a Shi'ite mosque.
Witnesses described body parts, apparently of a suicide bomber, in the aftermath.
A resident of the city reached by telephone said there were believed to be no casualties there apart from the attacker, as worshippers had already gone home to break their fasts. Civil defense forces were cleaning up the area and police were investigating, the resident said.
A video circulating on social media and purporting to show the aftermath of a Qatif blast showed an agitated crowd on a street, with a fire raging near a building, and a bloody body part lying on the ground. Reuters could not immediately verify the video.
Hours earlier a suicide bomber was killed and two people were wounded in a blast near the US consulate in the kingdom's second city Jeddah.