BAGHDAD — A multiple bombing that killed at least 33 people at a campaign rally for a militant Shiite group likely unleashed a series of apparent sectarian attacks in Iraq, signaling the start of a new wave of Sunni-Shiite bloodletting ahead of elections next week, security officials said Saturday.

An al-Qaeda breakaway group, the Islamic State of Iraq and the Levant, claimed responsibility for Friday’s attack on the Baghdad rally, which drew about 10,000 backers of Asaib Ahl al-Haq.

It said on a militant website that the bombings were to avenge what it called the killing of Sunnis and their forced removal from their homes by Shiite militias. The authenticity of the claim could not be independently verified.

A senior Asaib Ahl al-Haq official said the 33 dead included 10 group members who had fought in the Syrian civil war. Members of the Iranian-backed group, like Lebanon’s Shiite Hezbollah, have been fighting with forces loyal to President Bashar Assad, a member of an offshoot Shiite sect. The Islamic State fights with Sunni rebels trying to oust Assad.

Bomb attacks like Friday’s are not uncommon in Iraq, but targeting a gathering by a militant Shiite group raises the stakes in Iraq’s deadly Sunni-Shiite rivalry and had been expected to spark retaliation.

Several hours after the Baghdad bombing Friday, a senior Sunni politician in the mostly Shiite southern city of Basra was shot dead in what appeared to be a revenge attack. On Saturday, police found nine bodies, some bullet-riddled, in several Sunni and Shiite districts of the Iraqi capital, security officials said. The bodies could not be immediately identified.

Other bodies have been found in similar attacks reminiscent of the worst days of Iraq’s sectarian violence between 2006 and 2008. Then, Baghdad woke up almost daily to news of bodies found shot, decapitated or drilled.

Also Saturday, gunmen in a speeding car opened fire on a group of civilians in the mixed al-Amil neighborhood of western Baghdad, killing two people and wounding three. The security officials said the shooting took place in one of al-Amil’s Sunni sections.

In the afternoon, police said a bomb exploded inside a small restaurant in the mostly Shiite al-Nasir district in the eastern suburbs of Baghdad, killing four people and wounding 11.

Medical officials confirmed the casualty figures from the attacks.

Last year, the death toll in Iraq climbed to its highest levels since the violence between 2006 and 2008. The United Nations says 8,868 people were killed in 2013, and more than 1,400 people were killed in the first two months of this year alone.

Friday’s rally was held under heavy security, with hundreds of the group’s militiamen and veterans of the Syria war in charge. The 10 Syria war veterans killed were among scores of militiamen in green military fatigues patrolling the rally, said the Asaib Ahl al-Haq official. He and other officials spoke on condition of anonymity as they weren’t authorized to speak to journalists.

The rally was held to introduce Asaib Ahl-Haq’s candidates in Wednesday’s election, but a speech given by the group’s leader, Sheik Qais al-Khazali, carried heavy sectarian undertones with ominous threats. It included several references to the civil war in Syria and the conflict in the mainly Sunni province of Anbar west of Baghdad, where government forces are fighting Sunni militants in control of parts of two cities there, including the provincial capital, Ramadi.

“They fight Iraq’s enemies there on the land of Syria,” al-Khazali said, alluding to his fighters in Syria.

Asaib Ahl al-Haq started off as a Shiite militia nearly a decade ago. It was blamed for several attacks on US forces in Iraq, including a spectacular kidnapping of US soldiers in the Shiite holy city of Karbala in 2007. More recently, it has assumed a growing political role just as the country was experiencing the resurgence of Sunni-Shiite violence.

More than 9,000 candidates are taking part in Wednesday’s election, the fourth such vote since Saddam Hussein’s 2003 overthrow, and will vie for 328 seats in parliament. Parts of the Sunni-dominated Anbar province won’t take part in the election due the violence there.

The government of Shiite Prime Minister Nouri al-Maliki, meanwhile, announced a weeklong national holiday to coincide with the elections. It did not say why it extended a previously announced three-day holiday that was to start Tuesday. Authorities in past nationwide elections have declared extended holidays, chiefly to empty the streets and allow security forces faster access to attack sites.

Copyright 2014 The Associated Press.