Pete Buttigieg is leading the Iowa caucus with 62% of the precincts reporting, with former vice president Joe Biden falling back to fourth spot.
The former South Bend mayor received 26.9% with Bernie Sanders close behind with 25.1%, according to partial results from the first vote to pick a Democratic White House candidate.
Elizabeth Warren was third on 18.3% and Joe Biden fourth on 15.6%.
The day was marred by a long delay due to what Iowa’s Democratic Party called “inconsistencies in the reporting of three sets of results”.
A coding error in the app used by the party has been blamed for the delays.
Spokeswoman Mandy McClure said: “This is simply a reporting issue, the app did not go down and this is not a hack or an intrusion.”
Earlier, Iowa Democrats met at more than 1,600 schools, community centres and other public locations to choose their preferred candidate from a field of 11 contenders.
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Vermont senator Bernie Sanders and Mr Biden had been front runners ahead of what is the nation’s first presidential contest before November’s election.
While waiting for the results, the candidates delivered what should have been post-election speeches.
Mr Biden said: “We feel good about where we are, so it’s on to New Hampshire”, adding that he would “walk out of here with our share of delegates”.
Mr Sanders said he had “a good feeling we’re going to be doing very, very well here in Iowa” once results were posted.
He said: “Today marks the beginning of the end for Donald Trump.”
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