Democrat Hillary Clinton held a narrow lead in opinion polls over Republican Donald Trump as millions of Americans turned out on Tuesday to vote for the next president after one of the most negative campaigns in US history.
In a battle that focused on the character of the candidates, Ms Clinton, 69, a former Secretary of State, Senator and First Lady, and Mr Trump, 70, a New York businessman and former reality TV star, made their final, fervent appeals to supporters late on Monday to turn out the vote.
Ms Clinton led Mr Trump by 44 percent to 39 percent in the last Reuters/Ipsos national tracking poll before election day.
A Reuters/Ipsos States of the Nation poll gave Ms Clinton a 90 percent chance of defeating Mr Trump and said she was on track to win 303 electoral college votes out of 270 needed.
World financial markets were closely watching the outcome of election, with stocks up slightly on cautious expectations of a Ms Clinton win. The dollar and bond yields slipped, while gold inched up. US stocks had soared on Monday as investors bet on Ms Clinton, seen as the candidate more likely to maintain the status quo.
Polls begin to close at 7pm eastern time on Tuesday (1pm NZ time on Wednesday), with the first meaningful results due about an hour later.
US television networks called the winner of the 2008 and 2012 presidential elections at 11pm (5pm NZ time) or shortly after.
Mr Clinton cast her ballot at an elementary school near her home in Chappaqua, New York early on Tuesday morning.
"It is the most humbling feeling. I know how much responsibility goes with this," Ms Clinton said.
"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."
Mr Trump, voted alongside his family at a school in Manhattan, began election day with a call to the Fox & Friends morning news show.
"It's been a beautiful process. The people of this country are incredible," Mr Trump said of the election. However, he added, "If I don't win, I will consider it a tremendous waste of time, energy and money."
More than 40 million voters cast ballots before election day in early voting around the country.
Mr Trump and Ms Clinton were seeking to become the 45th president of the United States and the successor to Democrat Barack Obama, who served two four-year terms in the White House and is barred by the US Constitution from seeking another term.
Ms Clinton is aiming to become the first US woman president after spending eight years in the White House as First Lady from 1993 to 2001 before serving as a US Senator from New York and as Obama's Secretary of State.
Mr Trump was expected to draw support heavily from white voters without college degrees. Ms Clinton was likely to draw support from college-educated voters and Hispanic and black voters.
Major bookmakers and online exchanges were confident she would win. Online political stock market PredictIt put her chances on Tuesday of capturing the White House at 80 percent, down 2 percentage points from Monday.
Trump advisers say the level of his support is not apparent in opinion polls and that they believe the real estate developer is in position for an upset victory along the lines of the Brexit vote in June to pull Britain from the European Union.
"We have seen enormous momentum," Trump deputy campaign manager Dave Bossie said.
Ms Clinton has vowed to largely continue the policies of Obama and to overcome income inequality among Americans, with an unremitting divide between the rich and poor.
Mr Trump, launching his first bid for elected office after decades as a public figure, has positioned himself as an agent of change and has vowed to crack down on illegal immigration and end trade deals he says are harming US workers.
Majorities of voters in opinion polls have viewed both candidates unfavourably.
Victory in US presidential elections is earned not by the popular vote, but by an Electoral College system that awards the White House on the basis of state-by-state wins, meaning a handful of states where the race is close assume an outsized importance.
An early indicator of who might prevail could come in North Carolina and Florida, two must-win states for Mr Trump that were the subject of frantic last-minute efforts by both candidates.
Races in both those states were shifting from favouring Ms Clinton to being too close to call, according to opinion polls.
Democrats also are seeking to break the Republican lock on control of the US Congress.
A strong turnout of voters for Ms Clinton could jeopardise Republican control of the Senate, as voters choose 34 senators of the 100-member chamber on Tuesday.
Democrats needed a net gain of five seats to win control. All 435 seats in the House of Representatives were being contested. The House was expected to remain in Republican hands.
The marathon US election campaign has been one of the most negative in American history, with each candidate accusing the other of lacking the character and judgment to be president.