Separate attacks also killed seven U.S. soldiers in Baghdad and the southern city of Najaf.
The suicide bombings occurred in Karbala, one of Shiite Islam's holiest cities, and Ramadi, an insurgent stronghold west of Baghdad where U.S. and Iraqi forces have conducted operations to root out al-Qaida in Iraq fighters.
"The bomb was caused by a suicide bomber wearing an explosive belt, walking among people," Iraqi police Lt.-Col. Razak al-Taee said of the Karbala bombing, Reuters reported.
According to officials, the bomber detonated the belt, which was laced with ball bearings and a grenade, killing 51 people and wounding 138 others near the Imam Hussein shrine, a Shiite holy site 68 miles southwest of Baghdad.
Pedestrians loaded the wounded into cars and trucks, as television cameras showed scattered debris and blood at the scene, Reuters reported.
In Ramadi, a second bomber targeted a group of police and army recruits waiting in line near a recruitment center. The bomber blew himself up killing 60 people and wounding 70 others. The attack was the latest in a string of bombings and shootings aimed at discouraging Iraqis from joining the security forces that will replace the U.S. military.
Doctor Mahmoud al-Dulaimi of the Ramadi general hospital said U.S. soldiers had been among the victims of the attack, but had no other information, according to Reuters.
In Baghdad, five U.S. soldiers died when a roadside bomb hit their vehicle, the U.S. military said Thursday. A second roadside bomb in the southern city of Najaf killed two other American soldiers, bringing the number of U.S. troops killed in Iraq since the U.S.-led invasion in March 2003 to 2,189, according to military statistics.
An estimated 32,631 to 37,633 Iraqi soldiers and civilians were killed in the same period.
Elsewhere in Iraq Thursday, gunmen wounded the head of criminal intelligence in the Diyala province Lt.-Col. Adel Abdul Karem and killed three of his bodyguards. Three separate car bombs killed two people and wounded six in Baghdad. And a mortar attack seriously damaged a major gas pipeline in Bajwan, nine miles west of Kirkuk.
Thursday's violence came just a day after 58 people died in a wave of attacks around Iraq. Security officials believe the violence to be the work of Sunni insurgents hoping to create unrest as U.S.-supported Iraqi politicians work to build a functioning government following the Dec. 15 elections.
"These groups of dark terror will not succeed through these cowardly acts in dissuading Iraqis in their bid to form a government of national unity," Iraqi President Jalal Talabani said in response to the attacks.
Talabani and other political leaders will soon meet in Baghdad to discuss the creation of a coalition government they believe will help stem the bloodshed.