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US elections 2016: Voting begins in US presidential race US elections 2016: Voting begins in US presidential race
(35 minutes later)
The nationwide process of selecting candidates to run in November's US presidential race is under way in the state of Iowa.The nationwide process of selecting candidates to run in November's US presidential race is under way in the state of Iowa.
Voters have begun meeting - or caucusing - across the state in schools, churches and other venues.Voters have begun meeting - or caucusing - across the state in schools, churches and other venues.
Early indications based on entrance polling by US media suggest that Donald Trump and Hillary Clinton lead the Republican and Democratic races. With half the Republican votes counted, it appears that there is a three-way fight between Donald Trump, Ted Cruz and Marco Rubio.
Iowa kicks off a state-by-state contest over the coming weeks and months.Iowa kicks off a state-by-state contest over the coming weeks and months.
The Democratic race is also too close to call, with Vermont Senator Bernie Sanders mounting an unexpectedly strong challenge against the former first lady and Secretary of State, Hillary Clinton.
The third contender in that race, Martin O'Malley, is about to drop out of the contest, sources close to his campaign have told the BBC.
Follow our live updatesFollow our live updates
Campaigning intensified over the weekend as candidates toured the sparsely populated Midwestern state to court undecided voters.Campaigning intensified over the weekend as candidates toured the sparsely populated Midwestern state to court undecided voters.
Voting began at 19:00 local time (01:00 GMT on Tuesday) and the final results - a winner from each party - could be named within hours.Voting began at 19:00 local time (01:00 GMT on Tuesday) and the final results - a winner from each party - could be named within hours.
This first electoral test is seen as key because victory can boost campaign momentum as other states vote.This first electoral test is seen as key because victory can boost campaign momentum as other states vote.
According to interviews with voters who arrived early at caucus sites across the state, Mr Trump is in front in a three-way battle with two senators, Ted Cruz and Marco Rubio.
And Democrat Hillary Clinton appears to be slightly ahead of Vermont Senator Bernie Sanders, who has mounted an unexpectedly strong challenge against the former first lady and Secretary of State.
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.
All eyes on Iowa in unpredictable race
Supporters of the main candidates in Iowa gave very different reasons for their backing.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.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".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".
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Bernie Sanders supporter Mike Bates said "he's turning out the young voters. He's like the cool grandpa". Meanwhile Hillary Clinton backer Joan Pinnell said the former secretary of state could "personally understand the struggle that it is to be female".Bernie Sanders supporter Mike Bates said "he's turning out the young voters. He's like the cool grandpa". Meanwhile Hillary Clinton backer Joan Pinnell said the former secretary of state could "personally understand the struggle that it is to be female".
Iowa has an unusual election system called a caucus, which involves people gathering at private homes, schools and other public buildings across the state at 19:00 local time (01:00 GMT Tuesday).Iowa has an unusual election system called a caucus, which involves people gathering at private homes, schools and other public buildings across the state at 19:00 local time (01:00 GMT Tuesday).
Democratic voters divide themselves into groups based on their preferred candidate, but the Republican caucus process is more like a traditional ballot.Democratic voters divide themselves into groups based on their preferred candidate, but the Republican caucus process is more like a traditional ballot.
Turnout could be high, especially with a predicted snowstorm expected to strike after the vote.Turnout could be high, especially with a predicted snowstorm expected to strike after the vote.
The Iowa Republican Party said there could be more votes cast than the record set in 2012, when 120,000 people turned out.The Iowa Republican Party said there could be more votes cast than the record set in 2012, when 120,000 people turned out.
Democrats also expect a strong turnout, though not as many as the 240,000 people who caucused in the 2008 contest when Mrs Clinton was beaten into third by Barack Obama and John Edwards.Democrats also expect a strong turnout, though not as many as the 240,000 people who caucused in the 2008 contest when Mrs Clinton was beaten into third by Barack Obama and John Edwards.
Results are expected within hours. Correspondents point out that a sizeable number of Iowans in previous votes made their decision on the final day.Results are expected within hours. Correspondents point out that a sizeable number of Iowans in previous votes made their decision on the final day.
The last Republican winner in Iowa who won the party's nomination was George W Bush 16 years ago.The last Republican winner in Iowa who won the party's nomination was George W Bush 16 years ago.
Iowan law mandates that it be the first "state, territory, or any other group" to select delegates in the presidential nomination.Iowan law mandates that it be the first "state, territory, or any other group" to select delegates in the presidential nomination.
This first vote in Iowa will be followed in the weeks ahead by more ballots in the 49 other states plus US territories.This first vote in Iowa will be followed in the weeks ahead by more ballots in the 49 other states plus US territories.
Each party's nominee will be chosen by the summer, and the US will pick its next president in November.Each party's nominee will be chosen by the summer, and the US will pick its next president in November.