Japan's stock market had a miserable Monday, with the key stock index plunging more than 6 percent to its lowest close in more than a quarter century as investors grew increasingly gloomy about the outlook for the world's No. 2 economy.

Unnerved by the financial crisis fallout and the yen's recent surge, investors unloaded banks and exporters like Toyota and Sony.

The Nikkei 225 index shed 486.18 points, or 6.36 percent, to 7,162.90 — the worst closing level since October 1982. The benchmark has lost more than 20 percent since last Monday and nearly 40 percent in the last month. The broader Topix index posted an even steeper fall of 7.4 percent to 746.46.

Trading Monday was extremely volatile, with the Nikkei swinging from a 3 percent gain at one point in the morning to a retreat of as much as 6 percent in the afternoon.

A move earlier in the by Japanese Prime Minister Taro Aso to introduce measures to calm volatile stock markets failed to revive flagging sentiment.

Calling an emergency meeting of the Cabinet and ruling party officials, Aso urged steps including tighter controls on short-selling and expanding a government fund to recapitalize banks to as much as 10 trillion yen ($106.1 billion) from 2 trillion yen, according to Kyodo news agency.