The race to decide the US presidential election is on a knife edge with  Donald Trump beating Democratic rival Joe Biden in the vital battleground state of Florida.

With other competitive swing states that will help decide the election, including North Carolina, still up in the air, American look likely to go to bed without knowing who their leader will be.

Mr Trump and Mr Biden split the early US states in the White House race as expected, with conservative states like Alabama, Indiana, Kentucky and Tennessee going to the President and Democratic-leaning Massachusetts, Vermont, New York and Connecticut going to Biden, according to projections by television networks and Edison Research.

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But, with the exception of Florida, none of the approximately dozen battleground states that will decide the race had been settled as polls closed in a majority of US states, with close races developing in many of them.

Florida was the biggest and most fiercely contested battleground state, with both campaigns vying over its 29 Electoral College votes.

Mr Trump adopted Florida as his new home state, wooed its Latino community, particularly Cuban-Americans, and held rallies there incessantly.

Part of Mr Trump’s strength in Florida came from an improved performance relative to 2016 in the state’s counties with large Latino populations. Mr Trump’s share of the vote in those counties was larger than it was in the 2016 election.

Donald Trump supporters flock to the streets in Miami, Florida. Photo: Getty

Many younger Hispanics were ardent supporters of US Democratic Senator Bernie Sanders during the party’s primary campaign, but in opinion polls expressed little enthusiasm for Mr Biden, viewing him as too moderate and out of touch.

In the Miami area, Latinos are predominantly Cuban Americans, where generations of families have fled communist rule in Cuba.

Mr Trump’s messaging about Biden being a socialist seemed to be working with them and with Venezuelans there despite Mr Biden’s denials.

Edison’s national exit poll showed that while Biden led Trump among non-white voters, Mr Trump received a slightly higher proportion of the non-white votes than he did in 2016.

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Mr Biden, 77, still has multiple paths to the 270 Electoral College votes he needs to win without Florida despite having spent lots of time and money trying to flip the state that backed Mr Trump, 74, in 2016.

Mr Biden was neck and neck with Mr Trump in the battleground state of North Carolina, tied at 49.4 per cent with 86 per cent of expected votes counted.

In Ohio, another must-win state for Mr Trump, the President was on track to win with a lead of 50.5 per cent to 48.1 pre cent with 69 per cent of expected votes counted.

In Texas, Mr Biden narrowly led 49.6 per cent to 49 per cent with 72 per cent of expected votes counted.

Mr Trump monitored election returns with members of his family in the living room of the White House residence. “He’s calm, chilling,” said a source familiar with the scene.

Opinion polls show Trump, 74, is close enough in election battleground states, including Pennsylvania, Michigan and Wisconsin, that he could repeat the type of upset he pulled off in 2016, when he defeated Democrat Hillary Clinton despite losing the national popular vote by about 3 million ballots.

Joe Biden supporters watch presidential election returns in Miami. Photo: Getty

“I’m hopeful,” Biden told reporters in his home state of Delaware.

“What I’m hearing,” Biden added, “is that there’s overwhelming turnout, and overwhelming turnout particularly of young people, of women” and in some states of older black voters – groups expected to favour him.

“I think we’re going to have a great night,” Trump said in Arlington, Virginia where he thanked campaign workers. “But it’s politics and it’s elections, and you never know.”

“Winning is easy. Losing is never easy – not for me it’s not,” Trump added.

Ahead of Election Day, just over 100 million voters cast early ballots either by mail or in person, breaking records and prompting some experts to predict the highest voting rates since 1908.

Voters will also decide which political party controls the US Congress for the next two years, with Democrats narrowly favoured to recapture a Senate majority and retain control of the House of Representatives.

In anticipation of possible protests, some buildings and stores were boarded up in cities including Washington, Los Angeles and New York. Federal authorities erected a new fence around the White House perimeter.

-with AAP