Voters have begun meeting - or caucusing - across the state in schools, churches and even a gun shop, in a process to be repeated nationally.

Polls suggest that the race is tight for both parties, with Donald Trump and Hillary Clinton leading their fields.

Iowa kicks off a state-by-state contest over the coming weeks and months.

Over the weekend, the candidates toured the sparsely populated Midwestern state of Iowa in an attempt to court undecided voters.

Campaigning is expected to continue until voting starts at 19:00 local time on Monday (01:00 GMT on Tuesday).

While final rallies were held on Sunday, candidates have been holding smaller meetings and parties on Monday as they try to gain last-minute votes.

This first electoral test is seen as key because victory can boost campaign momentum as other states vote.

Analysis - Jon Sopel, BBC North America editor, Iowa

On the Democratic side, Hillary Clinton is facing a fierce challenge from the new kid on the block, Bernie Sanders, the 74-year-old senator from Vermont.

He describes himself as a democratic socialist, he wants to raise taxes, he doesn't argue with the description of himself as a grumpy old man - but he has been attracting thousands and thousands of people to his rallies, and millions and millions of dollars in donations.

His popularity seems to fly in the face of all conventional political wisdom. But then again, everything about this race so far has flown in the face of all the hoary old truths.

Among the wide Republican field, recent polling suggests that businessman Mr Trump has a comfortable, though not certain, lead over his main rival, Texas Senator Mr Cruz.

The Democrats' far smaller field - three candidates as opposed to 11 - appears to be more competitive.

Frontrunner Hillary Clinton has a commanding lead nationally, but in Iowa she is narrowly ahead of self-proclaimed "Democratic socialist" Bernie Sanders.

Supporters of the main candidates in Iowa gave very different reasons for their backing.

Suzanne Wood said Mr Cruz was someone who "knows how to argue for conservative values", citing his stance on gun rights and religion.

For Julian Raven, an artist and preacher campaigning for Donald Trump, the Republican has the "courage that is required to match up with the existential threats that we face as a western civilisation".

Source: BBC News