The London stock market has plunged more than 8% in the wake of the UK’s vote to leave the EU.
In the opening minutes of trade, the FTSE 100 index fell more than 500 points to 5,808.72.
Banks were especially hard hit, with Barclays and RBS falling about 30%.
Earlier, the value of the pound fell dramatically as the referendum outcome emerged. At one stage, it hit $1.3305, a fall of more than 10% and a low not seen since 1985.
The Bank of England said it was “monitoring developments closely” and would take “all necessary steps” to support monetary stability.
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“This is simply unprecedented, the pound has fallen off a cliff and the FTSE is now following suit,” said Dennis de Jong, managing director of UFX.com.
“Britain’s EU referendum has been a cloud hanging over the global economy for the past few months and that cloud has got very dark this morning.
“The markets despise uncertainty, yet that is exactly what they’re faced with this morning. The shockwaves are likely to reverberate for some time and the warning lights are flashing brighter now than ever.”
The FTSE’s slump was its biggest one-day fall since the collapse of Lehman Brothers in October 2008.
Traders worked through the night
As well as the banks, the housebuilding sector was also badly hit, with shares in Bovis Homes down more than 50%.
The impact of the vote was also felt in other European countries. The Paris and Frankfurt indexes were both down about 8%, while the Swiss central bank intervened on the money markets to steady the Swiss franc after it appreciated in value.
Oil prices have also fallen sharply in the wake of the referendum outcome, with Brent crude down 5.2%.
The price of Brent crude fell by $2.68 to $48.24 a barrel, its biggest fall since February. At the same time, US crude was down 5.4%, or $2.69, to $47.52 a barrel.
The price of gold jumped nearly 7% to $1,348.27 an ounce.
Before the results started to come in, the pound had risen as high as $1.50, as traders bet on a Remain victory.
But following early strong Leave votes in north-east England, it tumbled to $1.43 and then took another dive after 03:00 BST as Leave maintained its lead.
The move in sterling is the biggest one-day fall ever seen.
A weaker pound buys fewer dollars or other foreign currencies, which makes it more expensive to buy products from abroad. However, it should benefit exporters as it makes their goods cheaper abroad.
Against the euro, the pound dropped 7% to about €1.2085.
The euro also fell 3.3% against the dollar, its biggest one-day fall since the currency’s inception.
Media captionAsk Andy: What happens to the pound after the UK’s EU vote?
Currency traders say these moves are more extreme than those seen during the financial crisis of 2008.
“Never seen anything like it. These are once-in-a-lifetime moves, bigger than Lehmans and Black Wednesday,” said Joe Rundle, head of trading at ETX Capital.
“We’re waiting for the big money to crank into action over the coming days and even weeks, which will likely exert further downward pressure on sterling.”
David Tinsley at UBS said there would be “a significant rise in economic uncertainty” and that the Bank of England’s Monetary Policy Committee (MPC) was expected to take action, including interest rate cuts and an extension of its quantitative easing programme.
“We expect the MPC will cut policy rates to zero and make further asset purchases, in the first instance of £50-75bn, not later than February 2017,” he said.
Around midnight, sterling had risen to $1.50 after leading Leave campaigner Nigel Farage said it looked as though Remain had “edged” the vote.
But those gains were short-lived as the first results showed surprisingly strong votes to leave the EU