Traders work on the floor of the New York Stock Exchange (NYSE) in New York City, Nov 8, 2016. [Photo/VCG]

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

European shares looked set follow with losses of more than 4 percent as every new TV network projection in the US election showed the race to be far closer than anyone had thought, sending investors stampeding to safe-haven assets.

Sovereign bonds, the Japanese yen and gold surged while the Mexican peso went into near free-fall in chaotic trading.

"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 0440 GMT, Trump was leading Democratic rival Hillary Clinton by 35 Electoral College votes, with a tally of 244-209, with several key battleground states yet to be decided. It takes 270 to win.

US stock futures dived 5 percent, worse than the carnage that followed the British vote to leave the European Union in June that wiped trillions of dollars of value off world markets.

Investors fear a Trump victory could cause global economic and trade turmoil and years of policy unpredictability, 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 yet further.

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

Japan's top currency diplomat signalled Tokyo's readiness to intervene in currency markets if necessary as the surging yen threatened to snuff out its fragile economic recovery.

The scale of the scare was clear in the Mexican peso, which plunged more than 13 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. The risk of a global trade war likewise hammered currencies across Asia, with the Australian dollar leading the rout.

The story was very different against the safe-haven yen, with the US dollar shedding 3.8 percent to 101.24 yen. The euro jumped 2.5 percent to $1.1294.

Maximum uncertainty

Asian stocks skidded, with MSCI's broadest index of Asia-Pacific stocks outside Japan down 3.4 percent and the Nikkei off a savage 6 percent.

With voting completed in more than two-thirds of the 50 US states, the race was still too close to call in 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 had favoured Clinton as a status quo candidate who would be considered a safe pair of hands at home on the world stage. Analysts had no such certainly about Trump.

"With Brexit we had one bad day but this is different. This is what's scary about putting the most powerful position in the world in the hands of a man who many believe is temperamentally unstable," said Donald Selkin, chief market strategist at National Securities in New York.

"His tax cuts could open up a huge increase in the budget deficit and his trade sanctions could interrupt world trade. This could put us in a recession."

Sovereign bonds flew ahead, pushing yields on 10-year US Treasury notes down a huge 14 basis points to 1.73 percent, again the largest drop since Brexit.

Yields had briefly touched a six-month high around 1.8960 percent in early trade.

In commodity markets, safe-haven gold climbed 4.6 percent to $1,334 an ounce as the dollar slid.

Oil turned tail on concerns over the global economic outlook. US crude shedding $1.65 to $43.33 a barrel, while Brent fell $1.54 to $44.40.