Sterling crashed 10 percent to a 31-year low at one point and the euro also plummeted against the dollar, as the Brexit result caught markets by surprise.
European stocks went into free fall at the opening, mirroring a rout in Asian markets, after traders had banked in the run-up on Britain opting to remain in the EU based on polls and bookmakers' predictions.
London's benchmark FTSE-100 index plummeted 7.5 per cent after opening but began to recover some of the losses after British Prime Minister David Cameron said he would step down.
US stocks slipped more than two percent.
UniCredit Research economist Daniel Vernazza said British voters had snubbed warnings by "the overwhelming majority of expert economic opinion".
"Not surprisingly, this morning the referendum result has sent shock waves through global financial markets," he said in a note to clients.
After rallying above USD 1.50 at the time voting ended, the pound steadily crumbled to its lowest level since 1985, at USD 1.3229 at one point, before unwinding some losses.
But investors were still clearly wrong-footed by the vote outcome. European stock markets and the pound had gained steadily in the run-up to the referendum.
"It's scary, and I've never seen anything like it," James Butterfill, head of research and investments at ETF Securities, said in London. "A lot of people were caught out, and many investors will lose a lot of money," he told Bloomberg News.
Fears are also growing that other EU members will push for referendums, posing the biggest threat to the future of the grouping since its inception almost 60 years ago.
That worry pushed stock markets in the eurozone's weaker southern economies of Italy, Spain and Greece even lower than their core counterparts. Athens was more than 13 per cent down.
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