A suicide bomber blew himself up outside a Shiite mosque in eastern Saudi Arabia, while another blast occurred outside one of Islam's holiest sites, the Prophet's Mosque in Medina, a Saudi-owned television channel reported.
At least two people have been killed, according to reports.
Al-Arabiya news channel showed images of fire raging in a parking lot with at least one body nearby.
"Suicide bomber for sure. I can see the body torn apart," said a witness to the attack in the Shiite-populated city of Qatif.
Nasima al-Sada, another resident, told AFP that "one bomber blew himself up near the mosque", frequented by Shiites in downtown Qatif on the Gulf coast.
Pictures said to be from the scene circulated by residents showed a small fire burning in the street, severed limbs and what appeared to be a severed head.
The blast, near the end of the Muslim holy fasting month of Ramadan, was the second suicide bombing on Monday in the predominantly Sunni kingdom.
Another bomber blew himself up earlier the same day near the US consulate in the Red Sea city of Jeddah, injuring two security officers.
Since late 2014 a series of bombings and shootings claimed by the Islamic State jihadist group in Saudi Arabia have targeted minority Shiites as well as members of the security forces.
Most of the attacks have been staged in Eastern Province, home to the majority of Shiites in the Gulf state.
In January, a suicide bomber attacked a mosque in the oasis region of Al-Ahsa, killing four people before worshippers disarmed and tied up an accomplice who had fired on them.
Last October in the Qatif area, a gunman fired on faithful marking the Shiite commemoration of Ashura in the Qatif area, killing five before police shot him dead.
Ashura is one of the holiest occasions for the Shiite faith.
In June last year, four Shiites died preventing a suicide bomber from entering the hall of Al-Anoud mosque in Dammam city adjacent to Qatif.
Days earlier, 21 people were killed in another Shiite mosque bombing in Eastern Province.
Groups claiming affiliation with IS said they carried out those blasts and the Ashura shooting.
During Ashura in 2014, gunmen killed seven Shiite worshippers, including children, in the eastern town of Al-Dalwa.
The interior ministry said the suspects had links to IS, which regards Shiites as heretics.
(With inputs from AFP)