Republican Donald Trump has stunned the world by defeating heavily favoured Hillary Clinton in the race for the White House, ending eight years of Democratic rule and sending the United States on a new, uncertain path.

A wealthy real-estate developer and former reality TV host, Trump rode a wave of anger toward Washington insiders to defeat Clinton, whose gold-plated establishment resume includes stints as a first lady, US senator and secretary of state.

Worried a Trump victory could cause economic and global uncertainty, investors were in full flight from risky assets such as stocks. In overnight trading, S&P 500 index futures fell 5 per cent to hit their so-called limit down levels, indicating they would not be permitted to trade any lower until regular US stock market hours on Wednesday.

The Associated Press and Fox News projected that Trump had collected just enough of the 270 state-by-state electoral votes needed to win a four-year term that starts on January 20, taking battleground states where presidential elections are traditionally decided.

CNN reported Clinton had called Trump to concede concede the election.

A short time earlier, Clinton campaign chairman John Podesta told supporters at her election rally in New York to go home. "Several states are too close to call so we're not going to have anything more to say tonight," he said.

Victorious in a cliffhanger race that opinion polls had forecast was Clinton's to win, Trump won avid support among a core base of white non-college educated workers with his promise to be the "greatest jobs president that God ever created."

His win raises a host of questions for the United States at home and abroad. He campaigned on a pledge to take the country on a more isolationist, protectionist "America First" path. He has vowed to impose a 35 per cent tariff on goods exported to the United States by US companies that went abroad.

Both candidates, albeit Trump more than Clinton, had historically low popularity ratings in an election that many voters characterised as a choice between two unpleasant alternatives.

Trump, who at 70 will be the oldest first-term US president, came out on top after a bitter and divisive campaign that focused largely on the character of the candidates and whether they could be trusted to serve as the country's 45th president.

The presidency will be his first elected office, and it remains to be seen how he will work with Congress. During the campaign Trump was the target of sharp disapproval, not just from Democrats but from many in his own party.

Television networks projected Republicans would retain control of the US House of Representatives, where all 435 seats were up for grabs. In the U.S. Senate, the party also put up an unexpectedly tough fight to protect its majority in the US Senate.