The commission said its initial tally had been little more than a guess based on local estimates.
"Turnout figures recently announced represent the enormous and understandable enthusiasm felt in the field on this historic day," a commission statement said.
"However, these figures are only very rough, word-of-mouth estimates gathered informally from the field. It will take some time for the Independent Electoral Commission of Iraq to release accurate figures on turnout."
Commission spokesman Farid Ayar indicated that around eight million people may have voted, or about 60% of registered voters. That would still be more than many had expected.
A high turnout, especially in Sunni areas, would enhance the legitimacy of a 275-member parliament, which will choose Iraq's leaders and write a new constitution.
It could also help deflect criticism from Sunni Muslim groups that boycotted the polls.
Iraq's Election Commission said
its figure was a rough estimate
Leading candidates said reports from their monitors across the country showed more than two-thirds of registered Iraqis voted, despite bombings and attacks by fighters opposed to the presence of foreign troops in Iraq.
Planning Minister Mahdi al-Hafidh said monitors for his secular Independent Democrats list had noted a high turnout early in the day.
"Our monitors observed a 72% turnout. Iraqis are looking at these elections as an issue of dignity," he said.
Ahmad Chalabi, leader of the Iraqi National Congress, which is part of the main Shia electoral slate, said he was confident that nine million Iraqis had voted, with turnout significant even in Sunni areas.
He said United Iraqi Alliance monitors estimated turnout exceeding 50% in Salah al-din province, the mostly Sunni home region of ousted president Saddam Hussein.
"Millions of people created their own security and showed that they are more potent than terrorists," Chalabi said.
"The lack of preparations in the north is hurting
Mosul candidate Mishan al-Jibury
Turnout has been also overwhelming in Iraqi Kurdistan, Foreign Minister Hushiar Zibari, who is a Kurd, said.
"The turnout in Kurdistan is huge," he told Al-Arabiya television. "There is a rush for the Kurdish people to guarantee their representation in the National Assembly."
Mishan al-Jibury, a Sunni candidate from Mosul, said lower turnout in the Sunni areas was due to lack of security and functioning polling stations as well as calls for a boycott from Sunni groups hostile to the US military presence.
"The lack of preparations in the north is hurting my party," Jibury said, adding that he hoped flaws in the process would not repeat themselves in Iraq's next vote - an October referendum on the constitution to be drafted by the assembly.
"I can honestly say that this has been in general a fair and landmark dress rehearsal for democracy," he said.
The turnout in Kurdish areas is
believed to have been sizable
Speaking to Aljazeera from the northern city of Mosul, Mustafa Ibrahim, an independent Iraqi journalist, said the turnout in Mosul had been fair despite some problems.
"There was a fair attendance compared to the expectations of many in the city.
"In general, the election held in Mosul was a surprise to all as the number of voters was more than expected when considering the daily messages and posters threatening voters with death if they went to polling stations," Ibrahim added.
By contrast, heavily fortified polling centres were deserted and streets empty as Iraqis in the Sunni Muslim city of Samarra stayed home on Sunday, too frightened or angry to vote in the election.
"Nobody came. People were too afraid," Mudhafar Zaki, in charge of a polling centre in Samarra, said.
According to preliminary figures provided by a joint US and Iraqi taskforce that safeguarded the vote, fewer than 1400 people cast ballots in the city of 200,000.
The figure includes votes from soldiers and police, most of whom were recruited from the Shia south.