The first election since 2005 is to pick local councils in 14 of its 18 provinces and show whether Iraqi forces are capable of maintaining peace as US troops begin to pull back, almost six years after the invasion to unseat Saddam Hussein.
The last election took place amid a Sunni insurgency and was followed by a wave of sectarian slaughter between Iraq's once dominant Sunni Arabs and its majority Shi'ite Muslims.
Just under 15 million of Iraq's 28 million people have registered to vote for provincial councils that select regional governors.
Three Kurdish provinces are to vote separately and an election in Kirkuk has been put off because no one could agree on election rules.
Around 14,400 candidates are competing for 440 council seats in exuberant campaigning that has been made possible by a sharp drop in violence over the past 18 months.
Layers of campaign posters decorate the blast walls that divide Iraqi neighbourhoods, and balloons bearing political messages compete in the skies with airships used by US forces to spot mortar or rocket attacks by militants.
Rings of security
Voting stations, enveloped in rings of tight security to deter suicide bomb attacks by insurgents.
Thousands of Iraqi police and troops guarded the polling centres.
Cars were banned from cities to counter car bombs, airports and borders were shut and voters were being frisked for explosives-laden suicide vests and scanned for bomb residue.
Five candidates have been assassinated in the run-up to the election, three of them on Thursday.