Republican candidate Donald Trump has scored a series of shocking wins in battleground states including Florida and Ohio, opening a path to the White House for the political outsider and rattling world markets counting on a win by Democrat Hillary Clinton.

Key points: Polling places are now closed across the US

Polling places are now closed across the US Donald Trump is currently leading Hillary Clinton

Donald Trump is currently leading Hillary Clinton Republicans are expected to retain control of the House of Representatives

Mr Trump surged to wins in Ohio, Florida, Iowa and North Carolina.

Mrs Clinton has won Colorado and its nine electoral votes, and Virginia. Mrs Clinton is also projected to win Nevada.

However, Mr Trump is leading Mrs Clinton by 29 Electoral College votes, according to media reports that put him on 244 votes and Mrs Clinton on 215.

Mrs Clinton acknowledged a battle that was unexpectedly tight given her edge in opinion polls going into election day.

Mrs Clinton still has ways to reach 270 electoral votes but she will have to sweep the remaining battleground states including Pennsylvania, Michigan and Nevada.

Forty-one states have been called so far. Mrs Clinton has won 18 states to Mr Trump's 24.

There are 538 Electoral College votes allotted to the 50 states and the District of Columbia. It takes 270 votes to win.

Possibility of a President Trump sinking in

US correspondent Michael Vincent is at Trump headquarters, where the possibility of a Trump presidency is fast becoming a reality.

"This is a stunning result, not just for the political system here in the United States but potentially for the world's economy," he said.

"If Donald Trump wins this, Mexico and relations with the United States are going to be in for a very interesting time, let alone the rest of the world.

"NATO obviously in Europe — Trump has talked about allowing countries to get nuclear weapons. Let's see how that pans out. And also, of course, the trade war with China.

"There's a big, big sense here that people don't care. They just don't care. They want him to take back control of the system.

"This is — as one person said many months ago back in those early primary stages — this is the big middle finger to Washington and the establishment.

"It was always going be hard for Hillary Clinton to win essentially a third term for Barack Obama. George HW Bush won it back in 1988. Many said that that was really a third Reagan term, he was so popular.

"But this time around it is a stunning, stunning outcome, if [Mr Trump] does become the next president of the United States. And the implications for the world are only just beginning to sink in."

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With investors worried a Trump victory could cause economic and global uncertainty, the US dollar sank and stock markets plummeted in wild Asian trading.

Meanwhile, Washington Bureau chief Zoe Daniel is at Clinton headquarters in New York where supporters are trying to hold a shred of optimism.

"With three races too close to call in those battleground states, part of Hillary's firewall, and Pennsylvania now looking in doubt, I think there is a gradual sense of that optimism just completely dissipating," Daniel said.

"People have literally sat around in this venue on the floor, staring into space, some people are in tears. People are hugging each other.

"There's just a sense of pervasive shock. Not only that Hillary Clinton appears to have lost the election, but that it has been such a complete and utter rout.

"And the Democrats in here just don't understand what they're dealing with. I mean, I have been to dozens of Donald Trump events. I've spoken to probably hundreds of Donald Trump's supporters. We've had an ability to hear their arguments. These people haven't done that. And they just don't understand what's happened."

US Senate control on knife's edge in voting for Congress

Republicans will maintain their six-year control over the US House of Representatives, according to network projections, as the party also put up an unexpectedly tough fight to protect its majority in the US Senate.

The ABC and NBC television networks said Republicans, as widely projected in opinion polls, would keep control of the House, which has been in their hands since 2011.

Party dominance in Congress will be a crucial determinant of the policy-making outlook for the next president, whether it is Mrs Clinton or Mr Trump.

Democrats picked up their first Senate seat from Republicans as US Representative Tammy Duckworth, as expected, defeated Senator Mark Kirk in Illinois, according to the major TV networks.

With Ms Duckworth's victory, Democrats need to pick up four more seats from Republicans in order to win a majority in the Senate.

In Florida, incumbent Senator Marco Rubio — who ran unsuccessfully for the Republican presidential nomination earlier this year — was declared the winner and re-elected.

ABC/wires