TOKYO - Tokyo stocks plummeted 5.06 percent by the end of morning trade Tuesday after the collapse of US investment firm Lehman Brothers on Monday.

The benchmark Nikkei 225 dropped 618.67 points to 11596.09 points, the first time it finished at below 12,000 level in six months since mid-March.

The financial markets in Japan were closed on Monday as it was a national holiday.

Lehman Brothers Japan Inc. will file for bankruptcy protection with the Tokyo District Court, company sources said Tuesday.

Japan's central bank issued a statement on the day, saying it will carefully watch the development and take appropriate measures to stabilize the market.