Des Moines - Conservative Texas Senator Ted Cruz was declared the winner of the first Republican contest of the US presidential nominating contest on Monday.
Cruz came out four percentage points ahead of Donald Trump, the billionaire businessman who had led opinion polls heading into the Iowa caucuses.
Cruz had 28% to Trump's 24%, according to US broadcasters.
Florida Senator Marco Rubio finished in a strong third place with 23%, giving him a solid claim as the centre-right party establishment's favoured candidate.
Trump vowed to fight on.
"We will go on to get to the Republican nomination, and we will easily go on to beat Hillary [Clinton] or Bernie [Sanders], or whoever the hell they throw up there," Trump told supporters.
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Trump said he was "honoured" to finish with 24% of the vote, noting that advisors had told him at the start of his campaign that he could not compete in the state, where Republicans have traditionally favoured more conservative candidates.
Meanwhile, Republican Mike Huckabee has ended his bid for his party's presidential nomination.
"I am officially suspending my campaign. Thank you for all your loyal support," he wrote on Twitter.
Huckabee, a former Arkansas governor, finished the Iowa caucus with less than 2% of the vote on Monday. In the same contest in 2008 Huckabee finished first before ultimately losing the nomination to John McCain.
Democratic presidential hopeful Martin O'Malley also announced he will suspend his campaign after a poor showing in the first contest of the state-by-state US presidential nominating process, a source close to the campaign told dpa.
The Maryland ex-governor failed to reach even one percent of the vote in the Democratic caucuses in Iowa.
Clinton was locked in a tight contest race with Sanders, with Clinton at 50% and Sanders at 49%.