Iraq is voting in the first parliamentary elections since the withdrawal of US troops three years ago.
Polling began at 07:00 local time and closes at 18:00.
PM 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 past week alone.
Some 22 million Iraqis are registered to vote, with almost 50,000 polling stations open across the country.
There is a heavy security presence in the capital, with military helicopters on patrol.
The government has temporarily closed the airport and the main roads in and out of the city in an attempt to reassure voters.
Some voters face multiple searches before being allowed into polling stations.
While it is difficult to predict the outcome of the poll, Nouri al-Maliki is still expected to be a pivotal figure in the coalition-building process which will follow the election.
After casting his vote in Baghdad’s heavily-fortified Green Zone, Nouri al-Maliki said: “Our victory is certain, but we are waiting to see the size of our victory.”
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 9,000 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.
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