Roughly 24.5 million voters face a fragmented political landscape five months after the jihadists were ousted, with the dominant Shiites split, the Kurds in disarray and Sunnis sidelined.
Prime Minister Haider al-Abadi -- who took over as IS rampaged across Iraq in 2014 -- is angling for a new term, claiming credit for defeating the jihadists and seeing off a Kurdish push for independence.
But competition from within his Shiite community, the majority group dominating Iraqi politics, should divide the vote and spell lengthy horse-trading to form any government.
Whoever emerges as premier will face the mammoth task of rebuilding a country left shattered by the battle against IS -- with donors already pledging $30 billion.
Over 15 blood-sodden years since the US-led ouster of Saddam Hussein, disillusionment is widespread with the same old faces from an elite seen as mired in corruption and sectarianism.
More than two million people remain internally displaced and IS -- which has threatened the polls -- still poses a major security threat.
Overall, just under 7,000 candidates are standing and Iraq's complex system means no single bloc should get anything near a majority in the 329-seat parliament.
Votes in the Sunni heartlands once dominated by IS -- including Iraq's devastated second city Mosul -- are up in the air as traditional alliances have been shredded by the fallout of jihadist rule.
Political forces in the Kurdish community -- often seen as potential kingmakers -- are also in disarray after a September vote for independence backfired spectacularly.
A senior security official told AFP that some 900,000 police and soldiers are on high alert to protect the vote, with airports and borders shut for the day. Polling stations are open from 7:00 am to 6:00 pm (0400-1500 GMT) and initial results are expected in three days.
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