A suicide attacker and twin bombings Thursday targeted Shiites marking a somber religious ritual in Iraq, killing at least 41 people and wounding more than 100, officials said.

The ritual, known as Ashura, is observed every year over a 10-day period and has been marred previously by massive attacks by al-Qaeda and other Sunni extremists who see Shiites as heretics. This year, the attacks come amid an escalating campaign of violence by insurgents seeking to thwart the Shiite-led government’s efforts to maintain security.

The deadliest of Thursday’s attacks was in the town of al-Saadiyah, 90 miles northeast of Baghdad, where a suicide bomber struck a group of Shiites gathered for an Ashura event. The explosion killed at least 32 people and wounded 75, police said.

The Shiites at the Saadiyah gathering were re-creating the 7th-century battle of Karbala, a city in present-day Iraq. Ashura commemorates the death of the prophet Muhammad’s grandson, Imam Hussein, in that battle.

Earlier Thursday, two bombs exploded simultaneously near tents set up to provide food and drinks to Shiites pilgrims passing through Hafriyah, a town about 32 miles south of Baghdad, police said.

The Shiites were making their way on foot to Hussein’s gold-domed shrine in Karbala, 55 miles south of Baghdad, where authorities said more than 2 million pilgrims were expected to converge Thursday.

Ashura attracts hundreds of thousands of Shiites to holy sites across Iraq. Security forces have imposed tight security measures in and around Karbala, as well as other Shiite cities and Baghdad, sealing off areas where the Shiites, most of them dressed in black, passed through or stopped to rest.

Some Shiites in the processions ritually whipped their bodies with chains and knives in grief, drenching themselves in blood, which is part of Ashura.

On Tuesday, triple bombings struck a group of Shiites marking Ashura in the eastern city of Baqubah, a former al-Qaeda stronghold, 35 miles northeast of Baghdad, killing eight people, including two children.

Iraq has been hit by a surge in violence and insurgent attacks since April, when security forces cracked down on a Sunni protest camp in the north. The pace of the killings has soared to levels not seen since 2008.

More than 5,500 people died since April, according to U.N. figures.

There was no immediate claim of responsibility for the latest attacks, but suicide attacks and other bombings — especially against Shiites and Iraqi forces — are a favorite tactic of al-Qaeda’s local branch.

— Associated Press