The Nikkei 225 index at the Tokyo Stock Exchange opened up 1.74 per cent and widened the gain to 2.41 per cent or 203.35 points to 8,637.96 in the first 10 minutes of trading.
Stocks surged after six central banks -- for the eurozone, the United States, Japan, Britain, Switzerland and Canada -- opened up joint "liquidity support to the global financial system."
They were trying to boost confidence on markets wearied by failure to tackle Europe's deep debt crisis.
"The move intended to help short-term funding at financial institutions came as a surprise," said Yoshihiro Okumura, general manager at Chibagin Asset Management.
"This illustrates how urgent the situation might be," he told Dow Jones Newswires.
The euro fetched $1.3445 and 104.32 yen in early Asian trade, up from $1.3438 and 104.25 yen in New York late Wednesday.
The dollar was almost unchanged, trading at 77.58 yen.
The Dow Jones Industrial Average jumped 4.2 per cent to 12,045.68 in New York.
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