Such a body blow to global confidence could well prevent the Federal Reserve from raising interest rates as planned this year, and might even provoke a new round of emergency policy easing from all the major central banks.
KSE-100 index falls 2% in early trade on Brexit volatility
Risk assets were scorched as investors fled to the traditional safe-harbours of top-rated government debt, Japanese yen and gold.
Billions were wiped from share values as FTSE futures fell 7 per cent FFIc1, EMINI S&P 500 futures ESc1 5 per cent and Japan's Nikkei .N225 7.6 per cent. European stock markets were set to open more than 10 per cent lower STXEc1.
The British pound collapsed no less than 18 US cents, easily the biggest fall in living memory, to hit its lowest since 1985. The euro in turn slid 3.2 per cent to $1.1012 EUR= as investors feared for its very future.
Nearly complete results showed a 51.8/48.2 per cent split for leaving, setting the UK on an uncertain path and dealing the largest setback to European efforts to forge greater unity since World War Two.
Sterling sank a staggering 10.1 per cent at one point and was slumped at $1.3582 GBP=, having carved out a range of $1.3228 to $1.5022. The fall was even larger than during the global financial crisis and the currency was moving two or three cents in the blink of an eye.
"It's an extraordinary move for financial markets and also for democracy," said co-head of portfolio investments of London-based currency specialist Millennium Global Richard Benson.
"The market is pricing interest rate cuts from the big central banks and we assume there will be a global liquidity add from them in the next few hours," he added.
The shockwaves affected all asset classes and regions.
The safe-haven yen sprang higher to stand at 102.15 per dollar JPY=, having been as low as 106.81 at one stage. The dollar decline of 4 per cent was the largest since 1998.
That prompted warnings from Japanese officials that excessive forex moves were undesirable. Indeed, traders were wary in case global central banks chose to step in to calm the volatility.
One source told Reuters the Bank of England was in touch with other major central banks ahead of the market open there and the Bank of Japan Governor Haruhiko Kuroda it was ready to provide liquidity if needed to ensure market stability.
Britain unleashes global turmoil as it votes to leave EU
Other currencies across Asia and in eastern Europe as it woke up suffered badly on worries that alarmed investors could pull funds out of emerging markets. Poland, where many of the eastern Europeans in Britain come from, saw its zloty PLN= slump 7 per cent.
Recession fears
Europe's natural safety play, the 10-year German government bond, surged to send its yields tumbling back into negative territory and a new record low.
MSCI's broadest index of Asia-Pacific shares outside Japan .MIAPJ0000PUS slid almost 5 per cent, while Shanghai stocks .SSEC lost 1.1 per cent.
Financial markets have been gripped for months by worries about what Brexit, or a British exit from the European Union, would mean for Europe's stability.
"Obviously, there will be a large spill-over effects across all global economies if the "Leave" vote wins. Not only will the UK go into recession, Europe will follow suit," was the gloomy prediction of Matt Sherwood, head of investment strategy at fund manager Perpetual in Sydney.
Investors duly stampeded to sovereign bonds, with US 10-year Treasury futures TYc1 jumping over 2 points in an extremely rare move for Asian hours.
Yields on the cash note US10YT=RR fell 24 basis points to 1.49 per cent, the steepest one-day drop since 2009 and the lowest yield since 2012.
The rally did not extend to UK bonds, however, as ratings agency Standard and Poor's has warned it would likely downgrade the country's triple A rating if it left the EU.
Yields on 10-year gilts were indicated up 20 basis points at around 1.57 per cent GB10YT=TWEB, meaning higher borrowing costs for a UK government already struggling with a large budget deficit. Standard and Poor's has said it will strip the UK of its triple A credit rating.
Britain votes to leave EU in historic divorce
Across the Atlantic, investors were pricing in even less chance of another hike in US interest rates given the Federal Reserve had cited a British exit from the EU as one reason to be cautious on tightening.
"It adds weight to the camp that the Fed would be on hold. A July (hike) is definitely off the table," Mike Baele, managing director with the private client reserve group at US Bank in Portland, Oregon.
Fed funds futures <0#FF:> were even toying with the chance that the next move could be a cut in US rates.
Commodities likewise swung lower as a Brexit would be seen as a major threat to global growth. US crude CLc1 shed $3.00 to $47.11 a barrel in erratic trade while Brent LCOc1 fell as much as 6 per cent to $47.83 before clawing back to $48.18.
Industrial metal copper CMCU3 sank 3 per cent but gold XAU= galloped more than 6 per cent higher thanks to its perceived safe haven status.
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