The US dollar sank and stocks plummeted as mayhem came to world markets on Wednesday as investors faced the possibility of a shock win by Republican Donald Trump that could upend the global political order.

Every new TV network projection in the US presidential election showed the race to be far closer than anyone had thought, sending investors stampeding to safe-haven assets.

Sovereign bonds and gold surged while the Mexican peso went into near free-fall as stations gave North Carolina to Trump.

"Markets are reacting as though the four horsemen of the apocalypse just rode out of Trump Tower," said Sean Callow, a forex strategist at Westpac in Sydney.

"Or at least 3 of them - it might be 4 when the prospect of a clean sweep of Congress sinks in."

As of 0425 GMT, Trump was leading Democratic rival Hillary Clinton by 19 Electoral College votes, with a tally of 228-209, with several key battleground states yet to be decided. It takes 270 to win.

US stock futures recoiled more than 4.5 percent, matching the carnage that followed the British vote to leave the European Union in June that wiped trillions of dollars of value off global markets.

Investors fear a Trump victory could cause global economic and trade turmoil, discouraging the Federal Reserve from raising interest rates in December as long expected.

Fed fund futures were even starting to toy with the idea of a cut in rates next year and it was possible the Bank of Japan and European Central Bank might be forced to ease policy further.

South Korean authorities were thought to have intervened to steady their currency, and dealers were wondering if central banks globally would step in to calm nerves.

The scale of the scare was clear in the Mexican peso, which plunged more than 12 percent against the dollar in the biggest daily move in two decades.

"There's a lot of panic in the market, it is definitely an outcome it was not expecting," said Juan Carlos Alderete, a strategist at Banorte-IXE.

The peso has become a touchstone for sentiment on the election as Trump's trade policies are seen as damaging to its export-heavy economy.

But the story was very different against the safe-haven yen, with the dollar shedding 3.5 percent to 101.70 yen. The euro jumped 2.2 percent to $1.1265.

Asian stocks skidded, with MSCI's broadest index of Asia-Pacific stocks outside Japan down 2.5 percent and the Nikkei off nearly 4 percent.

With voting completed in more than two-thirds of the 50 US states, the race was still too close to call in Iowa, Michigan, Wisconsin, Pennsylvania and New Hampshire, states that could be vital to deciding who wins the presidency.

Fox News projected Trump had taken Florida and North Carolina, and projected Clinton would win Virginia.

Markets have tended to favor Clinton as a status quo candidate who would be considered a safe pair of hands at home on the world stage.

"In contrast, a Trump victory would trigger massive uncertainty that would likely undermine risk assets at least initially, which in turn could preclude a Fed rate hike this year," said Michelle Girard, economist at RBS.

Sovereign bonds flew ahead, pushing yields on 10-year US Treasury notes down a huge 13 basis points to 1.74 percent, the largest drop since Brexit.