Updated

Tens of thousands of mourners poured into the streets of eastern Saudi Arabia on Monday to attend a mass funeral for victims of last week's suicide bombing attack at a Shiite mosque that was claimed by the Islamic State group, attendees said.

Chanting religious slogans and carrying banners denouncing sectarianism, the mourners made their way with the victims' bodies along a route to a cemetery in the village of al-Qudeeh, in the oil-rich eastern Qatif region.

The Islamic State group claimed responsibility for Friday's attack and warned of more "black days" to come for Saudi Shiites. Saudi officials have confirmed the group was behind the blast, which officials said killed 21 people. It was the country's deadliest terrorist attack since 2004. King Salman has vowed to punish those responsible.

Like many ultraconservative Sunnis in Saudi Arabia, the IS group considers Shiites to be heretics. The kingdom's minority Shiites have long complained of discrimination in the country, which is ruled by the ultraconservative Wahhabi interpretation of Sunni Islam.

Monday's procession was a chance to mourn and to speak out.

"It was a normal funeral on one side, but on the other side it was a political rally," said attendee Fadel al-Hashim, a software engineer from Dammam. "There has always been some kind of encouragement toward hatred and sectarianism toward the minority Shiites. The message was: this has to stop."

Participants described a strong sense of solidarity, with volunteers handing out cold drinks along the procession route.

Mourners were bused in from surrounding towns and villages, including al-Ahsa, where eight Shiite worshippers were killed in a shooting attack in November that was also blamed on the IS group. Some came from as far as neighboring Bahrain and Kuwait.

"People were working together to make this funeral successful. They feel this should not be forgotten," said Salam Almarhoon, a law student from Safwa. "We see the sword on our neck."

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Associated Press writer Reem Khalifa contributed from Manama, Bahrain.

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