Global stock markets have sunk again despite central banks around the world announcing a co-ordinated effort to ease the effects of the coronavirus.
The Dow Jones index closed 12.9% down after President Donald Trump said the economy "may be" heading for recession.
London's FTSE 100 ended 4% lower, and other major European markets saw similar slides.
On Sunday, the US Federal Reserve cut interest rates to almost zero and launched a $700bn stimulus programme.
It was part of co-ordinated action announced alongside the eurozone, the UK, Japan, Canada, and Switzerland.
However, investors are worried that central banks now have few options left to combat the impact of the pandemic.
The new governor of the Bank of England, Andrew Bailey, has pledged to take "prompt action again" when necessary to stop the damage to the economy from the coronavirus pandemic.
David Madden, a market analyst at CMC Markets, said that while central bankers were trying to calm the markets, "in reality it is having the opposite effect".
"The radical measures have sent out a very worrying message to dealers, and that is why they are blindly dumping stocks."
In New York, steep falls as markets opened triggered another automatic halt to trading, which is meant to curb panic selling. Before last week, such halts, known as circuit breakers, had not been used in more than two decades.
But the sell-off continued after the 15-minute suspension, with the Dow losing nearly 3,000 points or 12.9%, its worst percentage drop since 1987.
The wider S&P 500 dropped 11.9%, while Nasdaq dropped 12.3%. All three indexes are now down more than 25% from their highs.
In London, firms in the travel sector saw big falls. Share in holiday firm Tui sank more than 27% after it said it would suspend the "majority" of its operations. BA-owner IAG fell more than 25% after it said it would cut its flight capacity by at least 75% in April and May.
The FTSE 250, which includes a number of well-known UK-focused companies, ended down about 7.8%.
All the main European share indexes fell sharply, though they later regained some ground. France's Cac 40 index fell more than 5.7% and Germany's Dax dropped more than 5.3%.
Earlier in Asia, Japan's benchmark Nikkei 225 closed down 2.5% and the Shanghai Composite in China ended the day 3.3% lower.
Oil prices, which have been shaken by a price war between exporters, fell again. Brent crude dropped by more than 10% to less than $32 a barrel while West Texas International crude fell more than 8% to less than $30 a barrel.
Hysteria overtaking common sense? Maybe not
Only a few weeks ago, the fear was that factories grinding to a halt in China's Hubei province could cause global growth to stumble briefly. Then it became clear that the economic pain was likely to be much more extensive.
Now, with tourism and leisure activities grinding to a halt and supply chains threatened, a widespread recession may be more likely than not. In other words, economic growth going into reverse for two consecutive quarters, which would mean cash flow emergencies for some businesses, possible corporate failures and rising unemployment.
The challenge for policymakers is to stop that downturn. Sunday's extravaganza by the Fed showed how central banks can go big. But interest rate cuts are of limited use; they won't tempt customers to go out and spend in Marseille or New York when the bars are closed, and flights cancelled.
So the markets are looking to governments for more targeted support and bailout packages. But even that won't calm nerves altogether. As banks enact their own workplace contingencies, what traders really want is a sign that the virus caseload has peaked, and then that the financial convalescence is on track.
On Sunday, the US central bank, the Federal Reserve, cut its interest rates by 100 basis points to a target range of 0% to 0.25% and said it would offer at least $700bn for support to the markets in the coming weeks.
The move comes as local officials across the US shut schools, restaurants and bars, sports leagues cancel tournaments, and retailers such as Urban Outfitters, Nike, and Gap announce hundreds of temporary store closures.
Speaking after the announcement, Fed chairman Jerome Powell said: "The virus is having a profound effect."
But stock markets dived as investors worried that the world's biggest central banks may now have very little ammunition left to deal with the effects of the coronavirus if the global economic climate continues to worsen.
"They [the Fed] pulled out whatever weapons they had and my sense is I think it may help initially but I don't think it goes much further because this is still a developing issue. They used up basically all their ammunition and we're down to sticks and stones," said Robert Pavlik, chief investment strategist at Slatestone Wealth.
Why should I care if stock markets fall?
Many people's initial reaction to "the markets" is that they are not directly affected, because they do not invest money.
Yet there are millions of people with a pension - either private or through work - who will see their savings (in what is known as a defined contribution pension) invested by pension schemes. The value of their savings pot is influenced by the performance of these investments.
So big rises or falls can affect your pension, but the advice is to remember that pension savings, like any investments, are usually a long-term bet.
Alongside the Fed, five other central banks - the Bank of England, the European Central Bank, the Bank of Japan, the Bank of Canada, and the Swiss National Bank - also announced measures to make it easier to provide dollars to their financial institutions facing stress in credit markets.
The move was designed to bring down the price banks and companies pay for US dollars, which has surged in recent weeks.
Andrew Sentance - a former member of the Bank of England's Monetary Policy Committee, which sets interest rates - told the BBC's Today programme that banks were acting to ensure enough credit was flowing.
"There was some criticism around the financial crisis that central banks didn't move quickly enough," he said. "I see this as a partly precautionary action for central banks to show that they are doing as much as possible to keep the wheels of the economy turning."
Mr Sentance added that any further cut to the base rate in the UK, to 0.10% for example, would be "symbolic, because it wouldn't have that much impact on companies or individuals".
In other developments on Monday:
- Strains continued in other parts of the market typically seen as less risky, with gold prices down more than 5% at one point
- The Bank of Japan eased monetary policy by pledging to buy risky assets at double the current pace and announced a new loan programme to extend one-year, zero-rate loans to financial institutions
- Shares in Australia recorded their biggest daily percentage fall on record, as the benchmark ASX 200 index dropped 9.7%
- The Reserve Bank of Australia said it "stands ready" to pump more money into the country's financial system
- New Zealand's central bank lowered interest rates by 75 basis points as it prepared for a "significant" hit to the economy.