Global stock indexes, including the UK FTSE 100, have fallen their most since the terrorist attacks of 11 September 2001 amid fears of a recession.
Investors remain worried about the state of the US economy
The FTSE 100 index tumbled 5.5% to 5,578.2, wiping £77bn ($149bn) off the value of its listed shares.
Indexes in Paris and Frankfurt slumped by about 7%, while markets in Asia, India and South America also dropped.
Investors questioned whether a recent plan to boost the US economy would be enough to avert a full-blown recession.
Dominique Strauss-Kahn, the head of the International Monetary Fund, said the global economic situation was "serious" and that all countries in the world were suffering in the wake of a slowdown in US growth.
Last week the US government announced a financial stimulus plan which would involve about $145bn in tax cuts to encourage spending.
US bond markets, which were closed for a public holiday on Monday, are to reopen on Tuesday and many analysts say they could see sharp falls after markets worldwide reacted negatively on Monday.
Francis Lun of Fulbright Securities in Hong Kong said the falls stemmed from disappointment that the US stimulus was "too little, too late" adding that investors felt "it wouldn't help the economy recover".
The worry is that tax breaks and spending measures will not be enough to boost consumer spending in the US, because deeper economic problems remain.
In particular, the slowing housing market and problems in the sub-prime sector - which lends to those with limited or no credit histories - has contributed to a slowdown.
"We're falling back into the crisis of confidence in the financial sector," said Hugues Rialan, of Robeco France.
"The banks have been reassuring the market over their exposure to US mortgage-related investments, but now we realise there is nothing reassuring about it," he said.
Finance firms were among the main fallers, with Dutch ING Group, Germany's Allianz and Swiss Re all falling about 10%, while Royal Bank of Scotland shed 8%.
FTSE100 - WORST DAYS
20/10/87 down 12.2%
19/10/87 down 10.8%
26/10/87 down 6.2%
11/09/01 down 5.7%
22/10/87 down 5.7%
Many shoppers are struggling under higher mortgage repayment costs, prompting default rates to surge, especially among sub-prime borrowers.
This has prompted banks to tighten their lending policies after losing huge amounts of investments linked to the US housing and mortgage markets.
The state of the US economy is crucial for many of Europe's and Asia's biggest companies because it is one of their biggest export markets.
FTSE100 - BEST DAYS
21/10/87 up 7.9%
13/03/03 up 6.1%
10/04/92 up 5.6%
15/10/02 up 5.1%
25/07/02 up 5.0%
Any slowdown in demand is likely to hurt corporate profit growth and push share prices even lower, analysts have warned.
But some analysts took comfort from the prospect of falling US interest rates.
"If interest rates are cut to the extent we and others expect, the likelihood is that today's share prices will look like silly values in 12 months' time, if not before," said Mike Lenhoff at Brewin Dolphin Securities.
Markets in China, India, South Korea, Singapore, Taiwan and the Philippines all fell.
In Mumbai, the main Sensex index fell 1,408 points, or 7.4%, adding to an 8% fall last week. Hong Kong's Hang Seng slumped 1,383.0 points, or 5.5%, to close at 23,818.9. Tokyo's main Nikkei 225 index fell 3.9%.
Australia's benchmark ASX 200 index closed down 2.9%, or 166.9, points at 5,580.4, which is its lowest level for a year.
It was also the 11th consecutive negative day for the index, the longest losing streak in more than 25 years.
"People are certainly nervous about a potential recession in the US spilling over to the rest of the world," said David Cohen at Action Economics.
So far this year, Japan's Nikkei has dropped 13%, the Hang Seng is down more than 14% and China's main Shanghai index has slipped almost 7%.