Democrat Hillary Clinton and Republican Donald Trump face the judgment of the voters as millions of Americans turn out on Election Day to pick the next U.S. president and end a bruising campaign that polls said favored Clinton.

A Reuters/Ipsos States of the Nation poll gave Clinton a 90 percent chance of defeating Trump and said she was on track to win 303 Electoral College votes out of 270 needed, to Trump's 235.

But Trump advisers said the level of his support was not apparent in the polling and believed the New York businessman was in position for an upset victory along the lines of the "Brexit" vote in June to pull Britain from the European Union.

"We have seen enormous momentum," said deputy Trump campaign manager Dave Bossie.

Financial markets brightened in reaction to the latest twists in what has been a volatile presidential campaign. Global stock markets and the U.S. dollar surged, putting them on track for their biggest gains in weeks.

Investors, who see Clinton as a known quantity, were buoyed by an announcement on Sunday by FBI Director James Comey that cleared Clinton of a cloud of controversy involving her use of a private email server while President Barack Obama's secretary of state from 2009 to 2013.

While opinion polls showed a close race, but tilting toward Clinton, major bookmakers and online exchanges were more confident of a Clinton victory. PredictIt put her chances of capturing the White House at 81 percent.

Reuters