Attacks against Shia, including a suicide bombing that ripped through a religious procession, 54 people and wounded 114, despite massive security deployed for one of the holiest days of their faith.
Thursday's 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 the repeated attacks that have marred the festival in previous years.
The suicide bomber, who was disguised in a police uniform, struck in a Shia-majority area of confessionally mixed Diyala province, north of Baghdad. It was the third attack of the day to target Shia.
Earlier, coordinated blasts in the town of Hafriyah, south of the capital, killed nine people, while twin bombings in the northern oil city of Kirkuk wounded five.
Worshippers were gathering inside a tent where Shias were performing rituals to commemorate the death of Imam al-Hussein.
Al-Hussein, a grandson of the Muslim Prophet Muhammad, was killed by the armies of the caliph Yazid in 680 AD and his death in Karbala has come to symbolise the split between Sunni and Shia Muslims.
Security measures have been stepped up in Iraq, with more than 35,000 soldiers and policemen currently deployed to Karbala, about 100km south of Baghdad, and surrounding areas, with security perimeters barring vehicles from entering the city while helicopters hover overhead.
Regional authorities expect two million pilgrims, including 200,000 from outside Iraq, will have visited the city of Karbala in the 10 days leading up to Ashoura, with all of the city's hotels fully booked.
On Wednesday, a car bomb targeted a Shia man distributing food in the northern city of Kirkuk, leaving eight wounded.
The violence against Shias is the latest in Iraq's worst unrest since 2008 and forced Nouri al-Maliki, Iraqi prime minister, to appeal to the US for help in the form of intelligence sharing and the delivery of new weapons systems in an effort to deal with the unrest.
Millions of Shias from Iraq and around the world mark Ashoura, which this year climaxes on Thursday, by setting up procession tents where food is distributed to passers-by and where pilgrims can gather.
Shias make up about 15 percent of Muslims worldwide. They are a majority in Iraq, Iran and Bahrain, and there are large Shia communities in Afghanistan, Lebanon, Pakistan and Saudi Arabia, and Syria.