European markets have continued to fall heavily amid speculation over the future of the Greek economy.
Stocks in both Paris and Frankfurt were down by more than 2% on Wednesday morning, following the downgrading of Greece's debt to "junk" status.
Spain's leading index is down 3.3%, while in Portugal - which also had its credit rating cut - shares fell 4%.
Meanwhile the euro fell to a fresh one-year low against the dollar before recovering slightly.
Overnight Japan's leading share index, the Nikkei 225, closed down more than 2.5% after steep falls in European stocks on Tuesday.
Greece's stock market regulators have also been forced to impose a ban on short-selling, amid concerns that Greek bank shares are being undermined by speculators.
Global shares have tumbled after the credit rating agency Standard and Poor's downgraded Greek debt to "junk" on Tuesday.
That means the rating agency views Greece as a much riskier place to invest, and increases the interest rate investors will charge the Greek government to borrow much-needed money on the open market.
On Wednesday, that interest rate hit 11.3% for 10-year Greek bonds - another all-time high for a eurozone country.
Concerns have intensified among some investors that the Greek crisis could spread to other vulnerable eurozone economies.
Yields on Spanish 10-year bonds have also reached their highest level - 4.27% - since the euro was launched.
Meanwhile investors are demanding an interest rate of close to 6% from Portugal - another relatively weak European economy.
"If yields rise much further Portugal may, like Greece, be in a position where [borrowing] on the open market becomes just too expensive," warned Jane Foley, research director at currency trader Forex.com.
Portugal's Prime Minister Jose Socrates spoke of "a speculative attack on the euro and Portuguese debt".
He said he would work with the opposition party to restore economic confidence in the country which also had its credit rating downgraded on Tuesday.
Gilles Moec, senior European economist at Deutsche Bank, said: "The market is now looking at every country with a lot of curiosity."
But he said other countries were not in the same dire straits as Greece.
"Portugal is clearly the most fragile country after Greece, but even so there is quite a lot of distance between [the two countries]," he told the BBC.
"The level of debt before the recession began was much higher in Greece. The immediate pressure on funding needs in Portugal is [therefore] not as dire."