Saudi-owned al-Arabiya TV also reported a suicide explosion near the security headquarters of the Prophet’s Mosque in Medina.
Witnesses earlier said a suicide bomber blew himself up outside a Shia mosque in Qatif on Monday evening without causing any other injuries.
They reported seeing body parts lying on the ground in the city’s business district.
“Suicide bomber for sure. I can see the body” which was blasted to pieces, a resident told the AFP news agency.
Nasima al-Sada, another resident, told AFP that “one bomber blew himself up near the mosque”, frequented by Shia Muslims in downtown Qatif, on the Gulf coast.
“We are in the last 10 days of Ramadan and those places are crowded because of that for Maghreb (sunset) prayers,” Khaled Batarfi, a Saudi Gazette columnist, told Al Jazeera.
Another witness told Reuters news agency that one explosion destroyed a car parked near a mosque, followed by another explosion just before 7pm local time.
Early on Monday morning, two security officers were injured as a suicide bomberblew himself up near the US consulate in the Red Sea port city of Jeddah.