"An IED (improvised explosive device) exploded inside the electoral office," said Haider al-Mullah, a candidate from Mutlak’s party, the National Dialogue Front, part of a secular coalition contesting the election.
"It's a covert action against the Iraqi nationalist forces and particularly Mutlak. We warned before that there would be attacks against us," Mullah added.
The interior ministry official said four other bombs targeted secular political party offices. All five attacks occurred between 9pm and 10pm (18:00-19:00 GMT).
The attacks came as tensions escalate between the Shia-led government and Sunnis, following the barring of hundreds of candidates from the elections because of ties to Saddam Hussein, the late president of Iraq.
Al-Mutlak was the number two candidate and top Sunni figure on Iyad Allawi's secular Iraqiya list, before being barred on Friday. Allawi was a former prime minister of Iraq.
Iraqiya said earlier on Saturday it was halting its campaign, after the decision to bar al-Mutlak and other members of its coalition.
"Iraqiya has decided to suspend its electoral campaign," Mayssun al-Damalduji, a spokeswoman for the party and a member of parliament, said.
Allawi and Jawad Bolani, also a secular list leader and currently the interior minister, are both trying to unseat Nuri al-Maliki, Iraq's prime minister, a Shia.
Al-Damalduji said Iraqiya was seeking a supreme court ruling as well as parliament to hold an emergency meeting to discuss the ban.
It has also requested a meeting with Jalal Talabani, the president, Iyad al-Samarrai, the parliament speaker, and al-Maliki "in the next three days to examine means of creating the best climate for the elections," she added.
"If we don't receive a reply, we will take a difficult decision," al-Damalduji said without further elaboration.
Official campaigning for the election started on Friday.
The electoral commission said on Saturday that 26 candidates were reinstated after being among the more than 500 banned from the vote, while 145 were disqualified, according to its final figures.
"The 26 are now able to campaign and we will inform their lists," Hamdiya al-Husseini, a senior commission official, told AFP news agency.
Husseini said among those eliminated were Mutlak and Dhafer al-Ani, another Sunni official.
A panel of judges had previously said barred candidates could stand on condition that their cases be examined after the polls and that they would eliminate them if they were found to be Baathists, but this ruling was reversed.
The vote, the second parliamentary ballot since Saddam was toppled, is seen as a test of reconciliation between the population’s Sunni minority and the Shia majority now represented by al-Maliki's government.