The caucuses began at 7 p.m. on Monday, bringing tens of thousands of Iowa's registered voters to churches, public libraries, and school gyms for discussions of their presidential preferences, reports Xinhua news agency.
Sitting President Donald Trump is expected to sail through the Republican caucuses, in which participants cast a vote to indicate their support, in the "Hawkeye State" that he won by 9.5 percentage points in the 2016 presidential election.
Unlike the Republican process, the Democratic caucuses require party members to show up to their precinct and physically move into designated parts of a room to express their preference for a certain candidate.
A crowded field of Democrats were vying for Iowa's 41 pledged delegates to the party's national convention on July 13-16 in Milwaukee, Wisconsin, a gathering for the party's delegates to choose their presidential nominee.
Though delegates of Iowa account for only 2 per cent of the 1,991 national delegates, a candidate needs to win the party nomination, while a strong performance in the state could inject strong momentum into his or her campaign.
Currently, there are 11 Democrats and three Republicans, including Trump, seeking the presidential nomination of their own parties. Seventeen Democrats and one Republican have dropped out of the race in this election cycle.
Meanwhile, a poll released hours before the caucuses showed that top candidates of the Democratic presidential primary field were locked in a tight race.
Focus on Rural America, a Democratic group, found that former South Bend, Indiana Mayor Pete Buttigieg was leading in the field with 19 per cent support, followed by Vermont Senator Bernie Sanders with 17 per cent.
Former Vice President Joe Biden and Senator Elizabeth Warren were tied on the third place at 15 per cent, while Senator Amy Klobuchar trailed behind them with 11 per cent, according to the poll.
"The top four candidates are in a virtual dead heat," the group said. "Anyone who says a particular candidate is best positioned to win is, to borrow from Vice President Biden, full of malarkey."
The poll also saw Buttigieg with the highest net favourability, 69 per cent. The rest of the top five candidates were not far behind, as each of them garnered at least 62 per cent.
According to the latest RealClearPolitics national Democratic primary polling average, Biden was leading the Democrat race with 27.2 per cent, followed by Sanders and Warren with 23.5 per cent and 15 per cent, respectively.
( With inputs from IANS )