Pete Buttigieg took the lead in the first batch of long-delayed results from the chaotic Iowa Democratic Party caucuses, with about 62 percent of precincts reporting.
Bernie Sanders was in second place, Elizabeth Warren in third, while former Vice President Joe Biden trailed badly in fourth place.
The results were released nearly 21 hours after Iowans poured into more than 1600 public locations to begin the five-month process of picking a challenger to Republican President Donald Trump.
In Tuesday's first results of state delegate equivalents, the data traditionally reported to determine the winner, Buttigieg, the former mayor of South Bend, Indiana, had 26.9 percent, Sanders had 25.1 percent, Warren 18.3 percent and Biden 15.6 percent. Senator Amy Klobuchar was fifth at 12.6 percent.
Sanders was ahead in the popular vote, which is not used to determine the delegates who will cast ballots at the Democratic National Convention in July.
Officials blamed inconsistencies related to a new mobile app used for vote counting for the unusual delay in Iowa, the state that traditionally kicks off a US presidential election campaign that culminates this year on 3 November.
The uncertainty enraged Democrats worried it would only strengthen Trump's bid for re-election and prompted some Democratic candidates' campaigns to question whether the results would be legitimate.
"As leader of the party I apologise deeply for this," Iowa State Party Chairman Troy Price told reporters. "We've been working day and night to make sure these results are accurate."
Republicans asked how Democrats could run the country if they could not conduct a caucus, while Trump mocked the Democrats on Twitter, calling the delay an "unmitigated disaster."
Before the results were released, campaign aides for Biden cited gross failures in the caucuses.
"What we're saying is there are some inconsistencies, that the process, the integrity, is at stake. And the Iowa Democratic Party needs to check that data, check it again, check it a third time, check it a fourth time, because it's important to get it right," Biden campaign senior adviser Symone Sanders told reporters.