Trading came to a virtual halt on US and European stock and commodities exchanges on Tuesday afternoon, as the full extent of the destruction became apparent.
European stock markets plummeted, with Frankfurt down 8.6%, London losing 5.7% - its biggest one day fall since 1987 - and Paris plunging 7.4%.
The New York Stock Exchange and the Nymex commodities exchange, just a few hundred metres from the devastated Trade Center towers, were evacuated, and trading was suspended "indefinitely".
The world's largest electronic stock market, the Nasdaq, suspended all its trading for the day, as did the American Stock Exchange.
"The American stock exchange, the Nasdaq stock market and the New York Stock Exchange, after consultation with the US Securities and Exchange Commission and in the light of the heinous attack on America, have decided to remain closed throughout Wednesday, 12 September," the New York Stock Exchange said in a statement.
It said there would be a further announcement on Wednesday about when the stock markets would open again.
Analysts said it was likely to be several days before markets restart full operations because of the devastation to buildings in the financial district.
On currency markets, the dollar plunged against the euro, the Swiss franc and the yen, while trading in oil was suspended in London, New York and Chicago, after prices rose sharply following the attacks.
The US central bank, the Federal Reserve, the Bank of Japan and the European Central Bank said they would stand by to provide cash to the banking system to ensure financial stability.
Financial workers
Many of those killed and injured in the New York attacks are likely to be financial workers, including, possibly, key figures from banking and trade.
Several top financial institutions had offices in the World Trade Center, including investment bank Morgan Stanley, which was the building's largest tenant with 25 floors, Switzerland's Credit Suisse Group, Germany's Commerzbank and Deutsche Bank and Bank of America.
The forty seven storey Salomon Brothers building next to the devastated twin towers of the World Trade Centre collapsed on Tuesday evening after catching fire.
A spokesman for Salomon brothers said the building, the former headquarters of the bank prior to its merger with Citigroup, had been evacuated at 1900GMT.
She said business would continue as normal at its London offices.
London markets
In Germany, the Frankfurt stock exchange was evacuated on Tuesday evening following a bomb threat.
London's stock exchange was also evacuated as a safety precaution, although screen-based trading continued.
A spokesman for the London Stock Exchange said trading would continue as normal on Wednesday.
"Exchange staff were moved from the exchange purely as a precaution," he said.
Trading continued at another, confidential, City location, he added.
The FTSE 100 index of leading shares closed down 287.7 points at 4746.
Meanwhile, many City workers were sent home early as the full horror of the disaster became apparent.
Markets in free fall
European markets went into freefall in the immediate aftermath of the attacks.
In Germany, where trading in all US stocks was suspended, the main stock markets closed 45 minutes earlier than usual.
At one point the Dax index of leading shares was down nearly 12%, but by the close it had recovered slightly to end down 396.6 points (8.6%) at 4,273.53.
In Paris, the Cac 40 index closed down 323.99 points (7.4%) at 4,059.75.
Wall Street's futures markets, meanwhile, also spiralled, with the trade in Dow Jones futures swinging between losses of 145 and 330 points.
One of Chicago's large futures exchanges, the Chicago Board of Trade, suspended trading.
Reversing its recent strength, the dollar fell below 199 Japanese yen, and lost more than one cent against the euro, trading at more than 91 US cents against the single European currency.
'Impossible to gauge'
Stock market historian David Schwartz told BBC News Online that it was impossible to gauge the economic fallout of the attacks from historic precedent.
"Pearl Harbor happened on a Sunday, and the Gulf War was a build-up over many months. There never has been a shock like this for the financial markets," he said.
Baring Asset Management director Michael Hughes believes governments and central banks would not hesitate to intervene.
"Hence the need to get confidence back into the system, not just for the sake of traders and markets, but for the sake of economic prosperity," he told the BBC's World Business Report.