TOKYO — The steep decline in U.S. stocks sent Asian stock markets tumbling sharply today as investors were rattled by concerns over an expanding global financial crisis.

Japan's benchmark Nikkei 225 index was down 5.3 percent to 11,560.66 in mid-afternoon trading, while Hong Kong's blue-chip Hang Seng Index shed 5.7 percent. Both markets — Asia's two biggest — had been closed for holidays on Monday, when news first broke about the dramatic events on Wall Street.

Across the region, markets were all deep in the red. South Korea's Kospi was down 5.4 percent, Taiwan's benchmark was off 4.7 percent and China's Shanghai index was down 3.2 percent.

Japan's central bank on Tuesday injected 2.5 trillion yen ($24 billion) into money markets and issued a statement vowing to take measures to maintain stability in the country's financial markets. Cabinet ministers, along with the central bank chief, were also holding an emergency meeting.

The dollar also got hit, falling to 104.43 yen early Tuesday afternoon in Asia from mid-107 yen levels before the weekend.

The early bloodletting in Asia followed a bleak Monday for world stock markets, which were hard hit after a double-fisted blow from Wall Street — news that Lehman Brothers had filed for bankruptcy and Merrill Lynch would be sold to Bank of America.

The Dow Jones industrial average fell more than 500 points in its largest point drop since after the Sept. 11, 2001, terror attacks.

In Europe, the FTSE-100 share index closed down 3.9 percent in London, the Paris CAC-40 slipped 3.7 percent and Germany's DAX 30 index of blue chips sagged 2.7 percent.

Asia's biggest stock exchanges in Japan, Hong Kong and South Korea were closed Monday for national holidays.

In Tokyo, Kyodo news agency reported that the Japanese unit of Lehman Brothers Holdings Inc. has requested bankruptcy protection at a Tokyo court after the 158-year-old firm filed for Chapter 11 bankruptcy in New York on Monday.

The company, crippled by $60 billion in soured real-estate holdings, was unable to find an investment partner to throw it a lifeline despite a flurry of last-minute negotiations over the weekend.

Share prices in Tokyo fell across the board, with banking issues taking a particularly hard hit in the wake of Lehman's collapse.

Downturns on the Australian and New Zealand markets were not as sharp as in some countries, in part because they had already absorbed some of the shock of Lehman and Bank of America news.