Political outsider Republican candidate Donald Trump is leading in the race to US presidency with a series of surprising wins in battleground states including Florida and Ohio.

CNN projections said Trump is leading with 238 electoral votes to Democratic candidate Hillary Clinton's 209.

The Trump lead sent shock waves across world markets that had counted on a win by Clinton as investors worried a Trump victory could cause economic and global uncertainty.

Trump surged to wins in Florida, Ohio, Iowa and North Carolina, and Fox News projected a win for him in Wisconsin. With voting completed in 49 of the 50 US states, he also narrowly led in Michigan and New Hampshire, edging him closer to 270 Electoral College votes needed to win the state-by-state fight for the White House, said a Reuters report.

Clinton still had ways to reach 270 electoral votes, but she would have to sweep the remaining battleground states including Pennsylvania, Michigan and Nevada, and pull off an upset win in Arizona.

The US dollar sank and stock markets slammed into reverse in wild Asian trade on Wednesday as every new exit poll in the US presidential election showed the race to be a nail-biter, sending investors stampeding to safe-haven assets.

Sovereign bonds and gold shot higher while the Mexican peso went into near free-fall as investors faced the real possibility of a shock win by Republican Donald Trump.

Gold climbed to $1,308.00 an ounce as the dollar slid. Oil turned tail, with US crude shedding $1.34 to $43.63 a barrel, while Brent fell $1.24 to $44.80.