Republican Donald Trump has conceded defeat to Texas Senator Ted Cruz in Iowa’s presidential nominating contest.
Ultraconservative Mr Cruz led the billionaire businessman by 27.7% to 24.3%, with 99% of precincts reporting results in the rural Midwestern US state.
Florida Senator Marco Rubio, viewed by many Republicans as a more mainstream alternative, nipped at Mr Trump’s heels in third place with 23.1%.
On the Democratic side, Hillary Clinton is locked in a dead heat with Vermont Senator Bernie Sanders in a race that’s too close to call.
Turnout was huge on both sides for the first-in-the-nation contest to pick candidates for November’s US White House elections.
Delivering his victory speech in Des Moines, Mr Cruz told supporters: “Tonight is a victory for courageous conservatives across Iowa and across this great nation.”
He said the result was a blow to the “Washington cartel”.
Mr Cruz’s win comes just four years after he rode a tea party wave to election in the Senate.
He focused his Iowa campaign message on evangelical voters, and two-thirds of Monday’s caucus-goers were born-again Christians.
Mr Trump, toning down his trademark bombast, told fans: “We finished second and I just want to say, I’m really honoured and I want to congratulate Ted.”
Rising star Mr Rubio told supporters: “Tonight we have taken the first step but an important step towards winning this election.”
The race now moves to next week’s New Hampshire primary, where Mr Trump has a stronger showing, according to opinion polls.
More than 180,000 Republican voters turned out on Monday night, compared with about 121,000 in 2012.
Long lines were reported at many caucus sites with many new voter registrations as Iowans gathered in schools, libraries and private homes.
At one Des Moines precinct, Post-it notes were used after ballot papers ran out.
The winners share the spoils of delegates to the party’s national convention in July when the presidential nominee is crowned.
Mr Cruz gets eight delegates, Mr Trump and Mr Rubio seven each, three for retired neurosurgeon Ben Carson and one each for Kentucky Senator Rand Paul and former Florida Governor Jeb Bush.
The crowded Republican field narrowed on Monday night as Mike Huckabee dropped out of the race.
Eleven of the party’s candidates remain in the race after months of rallies, televised debates and tens of millions of dollars of political advertising.