Americans on Nov 8 began voting for the 45th president of the United States. Democrat Hillary Clinton and Republican Donald Trump face voters on Tuesday as millions of Americans turn out on Election Day to pick the next US president and end a bruising campaign that polls said favored Clinton.

In a battle centered largely on the character of the candidates, Clinton, 69, a former secretary of state and first lady, and Trump, 70, a New York businessman, made their final, fervent appeals to supporters late on Monday to turn out the vote.

If she wins, Hillary Clinton could become America's first female commander in chief. A Reuters/Ipsos States of the Nation poll gave Clinton a 90 per cent chance of defeating Trump and said she was on track to win 303 electoral college votes out of 270 needed, to Trump's 235.

World markets braced for the outcome of one of the most contentious US presidential elections in history, with stocks up slightly on cautious expectations of a Clinton win. The dollar and bond yields slipped, while gold inched up.