The prosecutor's office in Al-Russafa, a district of eastern Baghdad, "arrested four people suspected of involvement in the arson attack on the electoral commission", Supreme Judicial Council spokesman Abdel Sattar Bayraqdar said in a statement.

"Three of them are police officers and the other is an employee of the electoral commission," he added.

The news of the arrests came after Iraqi Interior Minister Qassem al-Araji said there was "no doubt that (the fire) was a deliberate act".

The blaze occurred on Sunday ahead of a vote recount prompted by allegations of fraud during the election that saw a surprise victory for a populist cleric and Iraqi voters dumping the old guard.

Firefighters brought it under control several hours after it broke out at the warehouse in Al-Russafa.

Around 60 percent of the two million voters in Baghdad cast their ballots in the May 12 election in the district.

The extent of the damage caused to ballot boxes was still unclear but some officials have suggested most of them had been spared.

On Sunday an AFP reporter saw warehouse staff running out of the building as smoke billowed overhead, carrying blue and white ballot boxes to safety.

"Election material, including maybe ballot boxes, were burned but most of the ballot boxes were stored in another building and have been preserved," interior ministry spokesman General Saad Maan said on Sunday.

The fire erupted as nine judges were appointed to supervise a manual vote recount ordered by Iraq's outgoing parliament.

The legislature on Wednesday also sacked the nine-member independent commission that oversaw the polls.

The vote was won by populist Shiite cleric Moqtada Sadr's electoral alliance with communists, as long-time political figures were pushed out by Iraqi voters hoping for change in a country mired in conflict and corruption.

But the result was contested following allegations of fraud, namely by the veteran politicians led by parliamentary speaker Salim al-Juburi.