Attacks mostly against Shiites, including the suicide bombing of a religious procession, killed 44 people in Iraq Thursday despite massive security for one of the holiest days of their faith.

The bloodshed came as a flood of worshippers, including tens of thousands of foreign pilgrims, thronged the central shrine city of Karbala for the climax of Ashura, braving repeated attacks by Sunni militants that have marred the festival in previous years.

The suicide bomber, disguised in police uniform, struck in a Shiite-majority area of confessionally mixed Diyala province, north of Baghdad, killing 32 people and wounding 80, security and medical officials said.

It was the third attack of the day targeting Shiites.

Earlier, coordinated blasts in Hafriyah south of the capital killed nine people, while twin bombings in the northern oil city of Kirkuk wounded five.

Violence near Baghdad and in Diyala's provincial capital Baquba left three others dead.

Shiites from Iraq and around the world mark Ashura, which this year climaxed on Thursday, by setting up procession tents where pilgrims gather and food is distributed to passers-by.

An estimated two million faithful gathered in Karbala, site of the mausoleum of Imam Hussein, grandson of the Prophet Mohammed, whose death in the city at the hands of soldiers of the caliph Yazid in 680 AD lies at the heart of Islam's sectarian divide.

Tradition holds that the venerated imam was decapitated and his body mutilated.

To commemorate the occasion, modern-day Shiite devotees flood Hussein's mausoleum, demonstrating their ritual guilt and remorse for not defending him by beating their heads and chests.

In some cases they make incisions on their scalps with swords in ritual acts of self-flagellation.

Black-clad pilgrims packed the shrines of Hussein and his half-brother Abbas, listening over loudspeakers to the story of the battle in which Hussein was killed, as volunteers distributed food and water.

"I have been coming since I was young, every year, even during the time of the tyrant Saddam," said Abu Ali, a 35-year-old pilgrim from the southern port city of Basra, referring to the rule of the now-executed Sunni Arab dictator who savagely repressed Iraq's Shiite majority community.

Saddam Hussein barred the vast majority of Ashura commemorations, and the associated Arbaeen rituals, until his overthrow in the US-led invasion of 2003.

"I challenge anyone not to cry," the worshipper said, describing his emotions on taking part in Ashura ceremonies.

The commemorations, which also included a ritual run to Hussein's mausoleum and a reenactment of the attack that killed him, wrapped up in the early afternoon.

Provincial authorities expect two million pilgrims, including 200,000 from outside Iraq, will have visited Karbala in the 10 days leading up to Ashura, with all of the city's hotels fully booked.

Shiites make up about 15 percent of Muslims worldwide. They are a majority in Iraq, Iran and Bahrain, and there are large Shiite communities in Afghanistan, Lebanon, Oman, Pakistan, Saudi Arabia, Syria and Yemen.

Sunni militants linked to Al-Qaeda, who regard Shiites as apostates, often target them more during Ashura and Arbaeen, including by attacking pilgrims.

Security measures have been boosted, with more than 35,000 soldiers and police deployed to Karbala and surrounding areas.

Concentric security perimeters barred vehicles from entering the city, while helicopters flew overhead.

The violence against Shiites is the latest in Iraq's deadliest unrest since 2008.

It has prompted Prime Minister Nuri al-Maliki, himself a Shiite, to appeal to the United States for help in the form of intelligence sharing and new weapons systems.

Turkish Foreign Minister Ahmet Davutoglu offered Ankara's assistance during a recent visit to Baghdad.