A wave of car bombs ripped through mainly Shi'ite sections of Iraq's capital and in the southern city of Basra, killing at least 55 people and wounding scores more.
Monday's violence was extensive in Baghdad, where at least nine car bombs ripped through busy market places, crowded bus stops, and other areas of Shi'ite neighborhoods.
One particularly heavy explosion in the northern Shaab district of Baghdad killed at least 12 people and wounded 25.
Two others went off in the city of Basra, which is predominantly Shi'ite and one of the largest cities in Iraq.
The attacks come amid a spike in violence and high tensions between Iraq's Shi'ite majority and Sunni minority.
Bombings and other violence have killed more than 150 people in the past week, including 70 who died Friday in a series of bombings targeting Sunnis.
For months, Iraq's Sunnis have been protesting against Prime Minister Nouri al-Maliki, accusing his government of ignoring their needs and targeting them for arrest.
The unrest has raised fears of a return to the sectarian fighting that left tens of thousands dead in 2006 and 2007.
Violence has fallen from that peak, but the United Nations said 712 people were killed in April, making it the deadliest month in Iraq since June 2008.