Democratic nominee, Hillary Clinton remains the favourite with a slight lead in the overall vote.
Clinton started her day by casting her vote in Chappaqua, New York, where she and her husband, former president Bill Clinton, have lived since he left office in 2001.
“I know the responsibility that goes with this.
“So many people are counting on the outcome of this election and what it means for our country.
“I will do the best I can if I am fortunate enough to win today,’’ she said, as she greeted people waiting at the polling station to see her.
Clinton’s vice presidential candidate, Tim Kaine, also cast his vote a few hours earlier in his home state of Virginia.
However both campaigns kept up the pressure until the end.
Clinton’s campaign ticked down the minutes until the start of the polls with calls to vote and to build bridges, not walls, a dig at Trump’s promise to build a wall with Mexico to keep out illegal immigrants.
Trump’s Twitter feed was more dignified, but highlighted his final round of campaign stops ahead of the start of voting.
"Today we are going to win the great state of Michigan and we are going to win back the White House!" he wrote on his page.
Ordinary voters also took to twitter to make partisan appeals to any undecided voters or simply urge their fellow citizens to vote.
Meanwhile some citizens stayed nonpartisan.
There were reminders that restaurants like Krispy Kreme and White Castle were offering free food to those who voted on Tuesday.
Clinton remains the favourite, with a slight lead in the overall vote, which could translate into an even greater number of electors in the Electoral College.
According to opinion poll, the projections show her taking states with higher populations and, thus, more electors.
But the race marked by ugly rhetoric and personal attacks has been surprisingly close, especially since many Americans considered Trump’s campaign little over a novelty when it began in 2015.
Since then, he has built a strong campaign around people who felt they have been left behind by the current political system.
Voters are also electing members of the lower chamber House of Representatives and one-third of the upper chamber, the U.S. Senate.
Polling locations will continue to open across the U.S. all the way to Hawaii, the state farthest to the west.
Millions of voters already have cast their ballots in early voting, which is offered in 34 states and in Washington DC.
Report says the winner will become the 45th U.S. president on inauguration day, Jan. 20, and will succeed Obama, the nation’s first African-American president.