Iraqis are voting in the country's first parliamentary elections since the withdrawal of US troops three years ago.
Polling began at 4pm New Zealand time on Wednesday and was due to finish at 4am on Thursday.
Prime Minister Nouri al-Maliki is hoping to win a third term in office amid a growing insurgency in the west of the country.
Iraq is experiencing its worst unrest since 2008, with 160 people killed in the last week alone.
More than 20 million Iraqis are eligible to vote, with almost 50,000 polling stations open across the country.
The BBC reported there was a heavy security presence in the capital, with military helicopters on patrol.
The Government temporarily closed the airport and the main roads in and out of the city in an attempt to reassure voters.
While it is difficult to predict the outcome of the poll, Mr Maliki is still expected to be a pivotal figure in the coalition-building process which will follow the election.
His State of Law alliance, a Shia coalition, has largely avoided the fragmentation seen by other political blocs since the last election.
The campaign has so far been a violent one, with 50 people killed on Monday when soldiers, police and overseas citizens cast their votes.
One bomb struck a Kurdish political rally in the town Khanaqin, killing 30 people and wounding at least 50 others.
On Friday, at least 31 people were killed as a series of blasts targeted a Shia election rally in Baghdad. The Islamic State in Iraq and the Levant - an al-Qaeda offshoot - said it had carried out the attacks.
More than 9000 candidates are competing for 328 parliamentary seats.
There will be no voting in parts of Sunni-dominated Anbar province, where security forces still battle Islamist and tribal militants for control of the provincial capital Ramadi and nearby Falluja.