Republicans begin 2012 US election
Mitt Romney, Ron Paul and Rick Santorum led in preliminary results, but the race remains too close to call with votes held in more than 1,700 caucuses.
Six Republican contenders spent Tuesday making frantic final appeals to voters.
The caucus meetings are the first time voters have a say in the race to face Democratic President Barack Obama in November’s general election.
Tuesday evening’s contest launches months of caucuses and primary elections in 50 states, Washington DC and other territories, culminating in the Republican National Convention in August where the party nominee will be formally anointed.
About 120,000 voters gathered at more than 1,700 fire stations, schools, public buildings and private homes ahead of the official 20:00 EST (01:00 GMT) start to the voting.
An Associated Press poll of voters ahead of the start of the caucus start confirmed that former Massachusetts Governor Romney, Texas Congressman Paul and former Pennsylvania Senator Rick Santorum formed the top tier of candidates.
Former House Speaker Newt Gingrich, Minnesota Congresswoman Michelle Bachmann and Texas Governor Rick Perry are also in the race. Former Utah Governor Jon Huntsman is on the ballot, but did not campaign in Iowa.
Preliminary exit poll results from ABC News indicated independent voters – who are allowed to register as Republicans and participate in the caucuses – had voted heavily in favour of Mr Paul.
Evangelical Christians were split between Mr Santorum and Mr Paul, and the half of caucus-goers describing themselves as “very conservative” were tilting to Mr Santorum.
Also, ABC’s exit poll showed Mr Romney doing well among older voters – a significant chunk of the electorate.
Small town affairs
In the caucuses, voters are allowed to speak on behalf of the candidates they support, followed by a vote, the results of which are transmitted on to the media and to the state Republican party.
Many of the caucuses are tiny affairs held in small towns in each of Iowa’s 99 counties.
In Dawson, fewer than 20 voters gathered at a fire station to hold a caucus amid the fire trucks, reports BBC North America Editor Mark Mardell.
A hall in a school in Des Moines was overflowing with caucus goers, reports the BBC’s Steve Kingstone.
“It’s hard to spot a Bachmann fan, but other candidates all have support here,” he reports.
Iowa is not expected to settle the contest – John McCain, the eventual Republican nominee in 2008, came fourth in the state’s caucuses that year – but it will help shape the race for the White House.
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