Republican candidate Donald Trump has gained the edge over Hillary Clinton in the race for the key swing states that will decide the White House election.
He has won Florida, Ohio and North Carolina, while Democratic nominee Mrs Clinton took Virginia, ABC projects. Pennsylvania is too close to call.
The race is deadlocked in Michigan, which has not voted for a Republican White House candidate since 1988.
Markets lurched as Mrs Clinton's path to victory seemed less assured.
The US dollar and Mexican peso plummeted while the Dow futures dropped more than 650 points.
Mr Trump's Ohio win was a big boost to him, as no Republican has ever taken the White House without winning the Midwestern bellwether.
He also won Iowa, which last voted for a Republican in 2004.
The Republican nominee earlier racked up wins in the Midwest and South, while Mrs Clinton swept the Northeast, ABC News projects.
As expected, he has also been victorious in the Republican strongholds of Utah, Alabama, Kentucky, South Carolina, Nebraska, Indiana, West Virginia, Mississippi, Tennessee, Oklahoma and Texas, ABC projects.
And he is forecast to win Missouri, Montana, Louisiana, Arkansas, Kansas, North Dakota, South Dakota, Idaho and Wyoming - all solidly conservative states.
ABC tips Mrs Clinton to win the Democratic heartlands of California, Oregon, New York, New Jersey, Connecticut, Massachusetts, Maryland, Vermont, Delaware, Illinois, Rhode Island, Hawaii, Washington and District of Columbia, as well as New Mexico and Colorado.
A candidate must secure 270 of the 538 electoral college votes to declare victory.
Voting has now ended everywhere but Alaska, and full results are expected imminently.
In other developments:
- The North Carolina Board of Elections agreed to extend voting in eight precincts in Durham County amid long queues
- Failed Republican presidential candidate Marco Rubio won his bid for re-election in Florida's Senate race
- A gunman was found dead after allegedly killing one person and wounding three others near a polling station in the southern California city of Azusa
Mr Trump, a 70-year-old Manhattan real estate tycoon, and Mrs Clinton, who 69, would be the first US female president, voted earlier on Tuesday in New York City.
Mr Trump was booed as he arrived to cast his ballot at a school in Manhattan, alongside his wife, Melania, and daughter, Ivanka.
The two presidential hopefuls will spend election night in New York City, staging events barely a mile apart.
Mrs Clinton will address supporters at the Javits Centre in Manhattan, while Mr Trump holds an event at the Hilton Midtown hotel.
More than 5,000 police officers have been deployed across America's biggest city to keep order on election night.
Earlier on Tuesday, Mr Trump again declined to say whether he will accept the results.
"We're going to see how things play out," he told Fox News, while alleging there had been voting irregularities. "I want to see everything honest."
Some polling locations have reported equipment failures and long lines, but they appeared to be routine problems.
Mr Trump, who stoked conspiracies over the legitimacy of the democratic process during the campaign by claiming the vote would be "rigged", also filed an emergency lawsuit in Nevada.
The Republican's camp sued the Clark County Registrar of Voters, alleging the state had kept early voting stations open late.
But a judge rejected the request.
Voting kicked off just after midnight on Tuesday in the small New Hampshire village of Dixville Notch, where a mere seven votes were cast.
Mrs Clinton received four votes while two went for Mr Trump and one for libertarian candidate Gary Johnson.
All 50 states and Washington DC are voting across six different time zones.
Americans also have a chance to determine which party will control Congress.
All 435 seats in the House of Representatives were up for grabs, and ABC News forecasts it will remain in Republican hands.
But a third of seats in the Senate, where Republicans also hold sway, are in play, and Democrats hope to snatch control of that chamber.
A record number of Americans - more than 45 million - voted early by post or at polling stations.
Mr Trump, a populist political newcomer, provoked controversy on the campaign trail for comments about women, Muslims and a plan to build a wall along the US-Mexican border.
Mrs Clinton saw her campaign dogged by FBI investigations into whether she abused state secrets by operating a private email server during her time as US secretary of state.
Last Sunday, the law enforcement bureau cleared her once again of any criminality.
Mr Trump and Mrs Clinton are seeking to succeed Democratic President Barack Obama.
After two four-year terms in the White House, he is barred by the US constitution from running for re-election.