Britain's pound sterling lost as much as 10 percent of its value in a "flash crash", prompting concern about the vulnerability of the currency and other British assets.
The pound recovered from the initial plunge early on Friday, which took it as low as $1.1491 in Asia trading hours and was driven, dealers said, by the automated algorithmic computer trades that now dominate the global foreign exchange market.
But selling by European and US investors offset any bounce as first London then New York came on line, driving a 2 percent loss on the day and putting sterling on course for its biggest weekly fall since 2009.
However, the FTSE 100 index rose 1 percent while other European stock markets fell sharply.
There are growing worries about the impact that sterling's losses - and the Brexit nerves behind them - will have on domestic demand, inflation and economic growth in the years ahead.
Britain voted in June in a referendum to leave the European Union, a move known as Brexit.
Sterling's slump to its lowest in more than 30 years against the dollar this week in increasingly volatile trading has raised fears that Brexit could yet prompt a currency crisis like those of 1967, 1976 or 1992.
The FTSE 250 index of medium-sized UK firms, typically more dependent on the domestic market, fell for the third day running on Friday after hitting an all-time high earlier in the week.
Sterling has been falling steadily for a fortnight, as investors worry that the government's intention to prioritise immigration controls over access to the single market in exit talks will prompt deeper cuts to foreign investment in Britain.
So far, pension funds and other long-term fund investors have responded by buying UK shares and other assets while hedging the currency risk through options and other derivatives that allow them to sell the pound.
That may change if the currency proves too volatile to allow them to fund those trades, and measures of implied market volatility surged on Friday to more than 10 percent for durations out to a year.
Also on Friday, Europe's leaders pledged to allow no compromise on Brexit, underlining the dangers in advance for Theresa May's government.
French President Francois Hollande sent one of the strongest warnings yet that Britain will have to pay a heavy price for leaving the EU, while Jean-Claude Juncker, the European Commission chief, said the bloc must be "unyielding" in the face of Britain's demands.
Hollande called for "firmness" by the EU powers in Brexit negotiations to prevent other countries from following Britain's lead and leaving the bloc.
"There must be a threat, there must be a risk, there must be a price, otherwise we will be in negotiations that will not end well," he said in a speech on Thursday evening.
For his part, Juncker said that the 27 nations Britain was leaving behind must not give way easily.
US bond prices
Across the Atlantic, US government bond prices also fell sharply on Friday while gold had its worse day in nearly three years.
Investors are nervous about the timing and the pace of any increase in the super-low interest rates controlled by the Federal Reserve, and comments from central bank officials recently have added to their concerns.
US employers added 156,000 jobs in September, a decent gain that reflects a steady economy but also a sign that hiring has slowed from its robust pace last year.
The labour department says the unemployment rate rose to five percent from 4.9 percent, but mostly for a positive reason: more people felt confident enough to start looking for work, though not all of them found jobs.
As a result, more Americans were counted as unemployed.
Investors are also watching for effects from Hurricane Matthew as it hits the Florida coast, and assessing the significance of Friday's "flash crash" of the British pound.