The Associated Press will declare the victor based on the number of state delegate equivalents.
Glitches with a new mobile app caused confusion, they said, and some caucus organisers were forced to call in results for the state party to record manually, introducing human error and delays.
But Sanders was competing for progressive support with Warren, who put staff in the state far earlier than most of her opponents and has built a vast organization here.
"We don't know all the results, but we know by the time it's all said and done, Iowa, you have shocked the nation" he boasted. In the case of Iowa, those supporting a particular candidate must spend an hour or more standing up, in both a metaphorical and literal sense, for the one they are backing.
Des Moines County Democratic Chair Tom Courtney said he heard that in precincts across his county, including his own, a mobile app created for caucus organizers to report results to the party was "a mess". "So I'm going to tell you what I do know", Sen. "We know there's delays but we know one thing: We are punching above our weight".
Of the four main candidates Mr Buttigieg waited until last to speak publicly, suggesting he felt he had won, and delivered what appeared to be a victory speech.
"By all indications, we are going on to New Hampshire victorious", the former South Bend, Indiana, mayor told his supporters at Drake University after midnight. A small sampling of polls indicated that Buttigieg and Sanders were leading, with Warren, Biden and Klobuchar trailing behind.
Sen. Bernie Sanders (I-Vt.), who led in the pre-caucus polls, started with some wry humor. The Iowa Republican caucuses were canceled in 1992 and 2004.
Iowa has a unique voting process which requires a candidate at caucus sites to be viable by collecting at least 15% of the vote.
Unlike secret ballot voting, Iowa caucus-goers publicly declare their choice by standing together with other supporters of a candidate.
This year, Iowa will be sending 41 pledged delegates.
Earlier in the night, Iowa voters across the state cast their votes balancing a strong preference for fundamental change with an overwhelming desire to defeat Trump. They then will participate in "alignment", which allows supporters of eliminated candidates to choose again.
Democratic presidential candidate Michael Bloomberg says he has no regrets on skipping the Iowa caucuses. "That's why it's so distressing that the GOP has canceled so many primary elections and blocked challengers in many other states". The population of the United States is 73% white, according to Census figures. She said that the vote should be about more than beating US President Donald Trump, and that she hoped others would consider the broader policy issues that Sanders has long campaigned on.
Trump faced token opposition from former Congressman Joe Walsh and former Mass. Gov. Bill Weld.
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