At least 31 people were killed in Iraq and more than 40 others were wounded at a rally by a Shi'ite political organisation in Baghdad on Friday.
The BBC reports three bombs exploded as people left the rally for the Asa'ib Ahl al-Haq party. The first two were truck bombs and the third a roadside bomb.
The Islamic State in Iraq and the Levant, also known as ISIL, said it carried out the attack.
Asa'ib Ahl al-Haq is backed by Iran and supports President Bashar al-Assad of Syria.
More than 9000 candidates are seeking 328 parliamentary seats in Wednesday's election, which will be the first since the United States withdrew combat troops in 2011.
However, the BBC reports there will be no voting in parts of Anbar province, where security forces are still battling insurgents for control of Ramadi and Fallujah.
