SYDNEY: Carnage came to world markets on Friday as early voting returns suggested Britain was on the brink of leaving the European Union, threatening the existence of the entire bloc and its single currency.

Risk assets were scorched as investors fled to the safety of top-rated government debt, with FTSE futures off 6 percent FFIc1 and EMINI S&P 500 futures ESc1 down 2.5 percent.

At one stage the British pound had collapsed no less than 10 US cents, its biggest fall in living memory, while the euro slid 2 percent EUR= as investors feared for its future.

Sterling was last down a huge 5.8 percent at $1.4063 GBP=, having carved out a range of $1.3969 to $1.5022. The fall was even larger than during the global financial crisis and the currency was moving one or two cents in the blink of an eye.

"Sterling is getting smackadoodled," said Tim Kelleher, Head of FX Institutional Sales New Zealand for ASB Bank in Auckland. "While we thought it was all going to be "Remain", it's quite clearly not going to be as clear cut as that."

Early official results showed the margins were nail-bitingly tight but pointing to a "Leave". Betting firm BetFair estimated the probability of leaving as high as 74 percent.

Traders were particularly spooked by returns from Sunderland showing a large majority for the "Leave" camp and just a narrow win for "Remain" in Newcastle.

With 171 of 382 counting areas declared, the Leave camp was put at 51.3 percent against 48.7 percent for "Remain", according to Reuters calculations.

The tremors shook all asset classes and regions.

The safe-haven yen sprang higher to stand at 103.47 per dollar JPY=, having been as low as 106.81. That dragged the Nikkei .N225 down 3 percent and prompted warnings from Japanese officials that excessive forex moves were undesirable.

MSCI's broadest index of Asia-Pacific shares outside Japan .MIAPJ0000PUS slid 2 percent while Shanghai stocks .SSEC lost 0.5 percent.

UK bank shares listed in Hong Kong tumbled on the prospect of a destabilizing exit. HSBC (0005.HK) fell 4 percent while Standard Chartered (2888.HK) fell 6 percent.

FALSE HOPES?

Financial markets have been racked for months by worries about what Brexit, or a British exit from the European Union, would mean for Europe's stability.

Early opinion polls had favored the "Remain" camp and perhaps led to a false sense of complacency in markets. An Ipsos MORI poll put the lead at 8 points while a YouGov poll out just after polls closed found 52 percent of respondents said they voted to remain in the EU while 48 percent voted to leave.

Safe-haven bonds came back into favor as exits bets, with US 10-year Treasury futures TYc1 jumping 1.18 points. Yields on the cash note US10YT=RR fell 16 basis points to 1.58 percent.

Investors also priced in even less chance of another hike in US interest rates <0#FF:> 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.

Commodities likewise swung lower as a Brexit would be seen as a major threat to global growth. US crude CLc1 shed $1.64 to $48.47 a barrel in erratic trade while Brent LCOc1 fell $1.63 to $49.28.

Copper CMCU3 slipped but gold XAU= galloped higher thanks to its perceived safe haven status.