is it on a par with black wednesday?

marketwatch |  U.S. stock futures unraveled early Friday morning in New York, after the U.K. declared its intention to end its four-decade relationship with the European Union after a so-called Brexit vote.

Investors have been fretting that such an unprecedented decision to leave Europe’s trading bloc could destabilize the Europe’s fragile union and rattle markets.

Futures for the Dow Jones Industrial Average YMU6, -2.53%  fell as much as 700 points, but losses were paring from that ugly fall. 

Most recently, the Dow was down 485 points, or 2.7%, to 17,436. Futures for the S&P 500 ESU6, -3.26% tumbled 80 points, or 3.7%, to 2,028 and Nasdaq-100 futures NQU6, -3.24% cratered 173 points, or 3.9%, to 4,289.

Check out: MarketWatch’s live blog on Brexit

Markets began plunging after U.K. broadcasters BBC and ITV in the early hours of Friday morning local time forecast that the “leave” campaign had won the Brexit referendum and that the U.K. will sever its ties with the trading bloc it has been a member of since 1973. 

European stock-market indexes were being punished in the aftermath of the vote, with the Stoxx Europe 600 SXXP, -6.61%  off 8% at 318.81.

But moves in currencies, in particular, the British pound USDGBP, +6.7519% were the most pronounced. Sterling hit a low of $1.3230, a 10% slide from $1.4871 late Thursday in New York. But it has since pared some of that decline most recently at $1.3796. 

The stunning moves come after global markets rallied, betting that Britons would vote to reject Brexit, or a British exit from the EU. 

All three U.S. indexes soared into Thursday’s close, with the Dow DJIA, +1.29%  surging 230.24 points, or 1.3%, to finish at 18,011.07, while the S&P 500 SPX, +1.34% gained 27.87 points, or 1.3%, to close at 2,113.32. Meanwhile, the Nasdaq Composite Index COMP, +1.59%  climbed 76.72 points, or 1.6%, to close at 4,910.04.

Indications that the “leave” vote has won sets up global markets for the most volatile and frightening trading day since the market sank last August on fears about a slowdown in China’s stock market.