Twenty-one people have been killed and nearly 100 others injured when five bombs struck Shiite pilgrims during an important religious ritual for the Muslim sect.
Monday's attacks revealed the enormous security challenges that still beset Iraq as the US military leaves the country.
Shiite religious holidays such as the mourning period known as Ashoura are targeted every year by Sunni extremists and have become especially difficult tests for the US-trained Iraqi security forces, still struggling to protect their citizens.
The US military is rapidly shipping troops and equipment out of the country before a December 31 deadline to have all its forces out - then the entire responsibility for the nation's security will rest with Iraq's leaders.
Further underscoring the shortfalls, assailants just a week ago were able to even get a car bomb into the heavily fortified Green Zone that is the government's headquarters in the capital, Baghdad, and is home to many foreign embassies, prompting the US mission to severely limit the movement of its staff inside the zone. Iraqi officials said the bomb was meant for prime minister Nouri Maliki, but he was not in the area at the time.
Mr Maliki later confirmed that the bombing in the Green Zone was assassination attempt against him.
On Ashoura, the hundreds of thousands of pilgrims who walk to the southern holy city of Karbala from around the country in a show of religious Shiite fervour present a particularly easy target for Sunni militants who do not consider Shiites to be true Muslims.
After the fall of Saddam Hussein's Sunni-dominated regime in 2003, Shiites regained the right to express their beliefs freely, and since then the annual commemorations have drawn huge crowds despite the threat.
In Monday's first attack, a bomb exploded among Shiite pilgrims in Latifiyah, about 20 miles south of the capital, killing two of them and wounding three others.
Hours later, a car bomb exploded near a group of Shiite pilgrims in the town of Mahaweel as they were heading to Karbala, killing eight people and wounding about 56 other pilgrims. Mahaweel is about 35 miles south of Baghdad. Three more attacks against Shiite pilgrims in the capital killed 11 people and wounded 41 others, police said.