Dow futures plunged more than 4 percent and Japan's major index nosedived 6.1 percent, its largest drop in years. The Mexican peso likewise tumbled and investors looking for safe assets bid up the price of gold.
"Rightly or wrongly, markets are going to be concerned about a Trump victory, particularly given the potential consequences for world trade and its impact on many large companies in the U.S. stock market," said Ric Spooner, chief analyst at CMC Markets in Sydney.
During the campaign, Trump has threatened to greatly restrict immigration to the U.S., renegotiate the North American Free Trade Agreement, reject a Pacific Rim trade initiative known as the "TPP." He also has vowed to build a wall along the United States' southern border and force Mexico to pay for it.
Economists have said a Trump win could put the Mexican economy into contraction, and as results emerged Wednesday, the Mexican peso swooned 11.5 percent to 20.73 pesos to the dollar.
Shares had been higher early in Asia's Wednesday trading session but then turned sharply when Trump first gained the lead in the electoral vote count.
As of 11:30 EST (0430 GMT), Trump had taken 244 electoral votes to Hillary Clinton's 209. It takes a minimum 270 votes to win.
Pricing in a possible Trump victory, Dow futures were down 4.3 percent or 791 points at 17,496.00 and S&P futures had dropped 5.0 percent to 2,028.60.
Shares tumbled across Asia. Japan's Nikkei 225 index plunged 6 percent to 16,153.40 and Hong Kong's Hang Seng slumped 3.6 percent to 22,086.73. South Korea's Kospi shed 3.4 percent to 1,935.98, the Shanghai Composite index fell 1.3 percent to 3,106.23 and Australia's S&P ASX/200 in sank 1.9 percent to 5,156.60.
"Rightly or wrongly, markets are going to be concerned about a Trump victory, particularly given the potential consequences for world trade and its impact on many large companies in the U.S. stock market," said Ric Spooner, chief analyst at CMC Markets in Sydney. "Like Brexit, the rally over the last two days increases the downside potential if Donald Trump does win the election," he added, referring to Britain's unexpected vote to leave the European Union that shook world markets.
The price of gold, seen as a safe place for investors' money in times of uncertainty, soared 4.7 percent to $1,334.30 an ounce.
The election uncertainty also jolted currency markets, sending investors fleeing from the dollar. The greenback plunged 3.5 percent to 101.26 yen from 105.46 earlier in the day. The euro rose to $1.1293 from $1.1020.
Trump has threatened to renegotiate the North American Free Trade Agreement and vows to build a wall along the United States' southern border and force Mexico to pay for it.
Financial analyst Gabriela Siller of Banco BASE issued a forecast earlier Tuesday that a victory by Donald Trump could cause the rate to fall to 24 to the dollar next year and lead to a 3 percent economic contraction in Mexico.
Energy markets were also roiled. Benchmark U.S. crude futures lost $1.65, or 3.7 percent, to $43.33 a barrel in electronic trading on the New York Mercantile Exchange. The contract rose 9 cents to close at $44.98 a barrel on Thursday. Brent crude, used to price international oils, slid $1.35, or 2.9 percent, to $44.69 a barrel in London.
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