Habib Mahmoud, managing editor for the state-linked Al-Sharq newspaper in Qatif, said that the local Red Crescent authorities confirmed to him that 19 people had been killed and 28 wounded.
There was no immediate word from the Interior Ministry on casualties. The area is heavily populated by members of Saudi Arabia's Shiite minority.
Saudi Arabia's Interior Ministry initially reported that an explosion struck a mosque in the eastern province of Qatif after Friday prayers, without providing further details. It later confirmed that a suicide bomber who hid the explosives under his clothes was behind the attack.
Interior Ministry spokesman Major General Mansour al-Turki said in a statement that today's attack struck the Imam Ali mosque in a village called al-Qudeeh.
Also Read
A local activist, Naseema al-Sada, told The Associated Press by telephone from Qatif that the suicide bomber attacked worshippers as they were commemorating the birth of Imam Hussain, a revered figure among Shiites. She said the local hospital has called on residents to donate blood.
One person dressed in a white robe was being carried away on a stretcher.
Mahmoud, the newspaper editor, said the attacker stood with the worshippers during prayer and then detonated his suicide vest as worshippers were leaving the mosque.
Shiite residents in eastern Saudi Arabia have long complained of discrimination. They say that despite the region being home to most of the kingdom's oil reserves, their streets, buildings and infrastructure are in poor condition.
In 2011, Shiites in the east inspired by the Arab Spring uprising in neighbouring Bahrain took to the streets to demand greater rights. Police arrested hundreds of people and a counter-terrorism court sentenced an outspoken cleric, Nimr al-Nimr, to death.