Iraqi officials say at least 36 people were killed in bomb attacks against Shi'ite Muslims performing rituals for Ashura Tuesday.
Police say at least 23 people were killed and nearly 60 were wounded when a suicide bomber blew himself up at a Shi'ite mosque near the town of Mandali, close to the Iranian border.
In the nearby town of Khanaqin, at least 13 people were killed when a bomb exploded as Shi'ites marched toward a mosque.
In Baghdad, at least five Shi'ite pilgrims were killed when their car was ambushed.
Also in the capital, mortar shells slammed into a Sunni neighborhood, killing at least 10 people.
Meanwhile, thousands of Iraqi soldiers and police are deployed around the city of Karbala, where nearly two million Shi'ite pilgrims have gathered for Ashura.
Ashura marks the seventh century killing of the Shi'ite Imam Hussein, the grandson of the Prophet Muhammad.
Shi'ites believe Hussein was the prophet's heir. Hussein's death solidified Islam's split into rival Sunni and Shi'ite branches.
Earlier this week, near the Shi'ite holy city of Najaf, Iraqi troops backed by U.S. forces battled an Islamic cult that officials said was plotting to assassinate senior Shi'ite clerics.
Iraqi officials say more than 200 militants were killed during Sunday's fierce fighting that also involved U.S. airstrikes. About 100 militants were detained.
Separately, the U.S. military said one U.S. Marine died Monday during combat operations in western al-Anbar province, while a soldier was killed in an accident near the city of Nasiriyah, south of Baghdad.
Some information for this report was provided by AFP, AP and Reuters.