The time has come for America to decide between Donald Trump and Hillary Clinton. Follow live updates on the US election 2016 here.
America's future hung in the balance on Tuesday as millions of eager voters cast ballots to elect Democrat Hillary Clinton as their first woman president, or hand power to the billionaire populist Donald Trump.
As the world held its collective breath, Americans were called to make a historic choice between two radically different visions for the most powerful nation on Earth.
While Clinton has a slim lead in polls, no one was ruling out a victory by her Republican rival Trump — with the winner's name not expected to be known before 3 am GMT on Wednesday.
By 5 pm GMT, voting was underway in all 50 states and the capital Washington. In Virginia horse country, balmy south Florida, and busy Manhattan long lines snaked into the streets outside polling stations.
The time has come for America to decide between Donald Trump and Hillary Clinton. AP
"I'm excited. I can't believe I finally get to vote," said Jose Maria Molleda, 63, a new US citizen casting his ballot at a Presbyterian church in Clifton, Virginia, where a crowd of 150 gathered before dawn for the opening of polls in the swing state.
Katie Kope, another first-time voter in Staten Island, New York, was jubilant after casting her ballot for Trump and his promise to reclaim power from a corrupt Washington elite.
"I was kind of torn between the two but I don't trust Hillary, so that's what it came down to," said the 19-year-old.
An hour's drive north, a crowd of admirers chanted "Madam President" as Clinton and husband Bill voted near their home in Chappaqua.
"I'm so happy, I'm just incredibly happy," a beaming Clinton said as she emerged, shaking hands, mingling and chatting with the crowd.
"I know how much responsibility goes with this," said the 69-year-old former secretary of state. "So many people are counting on the outcome of this election, what it means for our country, and I'll do the very best I can if I'm fortunate enough to win today."
A few hours later it was Trump who rolled up to his voting station in Manhattan, casting his ballot alongside wife Melania in a school gymnasium.
"Right now it's looking very good," he told reporters — paying no heed to the crowd of protesters who welcomed him with chants of "New York hates you!"
Who wears the pants
A polling average by tracker site RealClearPolitics gave Clinton a 3.3-percentage point national lead, but Trump is closer or even has the advantage in several of the swing states that he must conquer to pull off an upset.
In must-win Florida, Clinton was already assured of the vote of 74-year-old Leonor Perez, who cast her ballot in the Cuban enclave of Hialeah near Miami.
"I voted for Hillary because it's time for a woman to wear the pants in this country," Perez said.
Clinton urged the country Monday to unite and vote for "a hopeful, inclusive, big-hearted America."
Trump pressed his message with voters who feel left behind by globalisation and social change, wrapping up with a flourish on his protectionist "America first" platform.
"Just imagine what our country could accomplish if we started working together as one people, under one God, saluting one American flag," the former reality television star, 70, told cheering supporters in Michigan.
'Test of our time'
In a kick-off midnight vote, the residents of tiny Dixville Notch, New Hampshire cast their traditional first-in-nation ballots with a total of eight votes — Clinton getting four, Trump two, and two votes going to others.
No full results or exit polls will be available before polling stations begin to close on the US East Coast from 7 pm (12 am GMT Wednesday), and it may be three or more hours after that before the direction of the race becomes clear.
Even then, questions remain. Trump has repeatedly claimed Democrats and the media are seeking to rig the race and said last month that he may not concede defeat if he thinks voting is unfair.
Asked at his voting location whether he would concede if networks call the election for Clinton, Trump said: "We'll see what happens."
Clinton has pushed a more optimistic vision, despite a wobble in recent weeks when the FBI reopened an investigation into whether she had put US secrets at risk by using a private email server — only to close the probe again on Sunday.
In a radio interview on the last night of the campaign, she said the matter was behind her, and she courted voters at her final rallies in Philadelphia with President Barack Obama and rocker Bruce Springsteen, and in North Carolina with pop diva Lady Gaga.
"Tomorrow, we face the test of our time," she declared in front of 40,000 people in Philadelphia.
"There is a clear choice in this election. A choice between division or unity, an economy that works for everyone, or only for those at the top; between strong, steady leadership, or a loose cannon who could put everything at risk."
'I will fight for you'
Trump meanwhile concluded a last-gasp tour of swing states by painting his rival as a corrupt creature of a discredited elite.
Promising to end "years of betrayal," tear up free-trade deals, seal the border and subject Syrian refugees to "extreme vetting", Trump told supporters in New Hampshire: "I am with you and I will fight for you and we will win."
Voters are also electing the entire 435-member House of Representatives, and candidates for 34 seats in the 100-member Senate, where Democrats are seeking to snatch control back from Republicans.
Trump's campaign spooked world markets seeking stability after the recent global slowdown.
Early Tuesday afternoon, the Dow Jones Industrial Average and the S&P 500 were each up about 0.6 percent.
Andy McLevey of London stockbroker Interactive Investor said investors were still treading with caution.
"Despite polls signalling a Hillary Clinton victory seems likely, it is still too close to call and we may see some jitters as the day progresses," he said.
With inputs from agencies