Tokyo's benchmark stock index plunged more than 3 percent Monday amid investor concerns about anti-Japan protests in China.
The Nikkei average of 225 selected issues closed down 432.25 points, or 3.8 percent, to end at 10,938.44 points -- its lowest point since Dec. 16, when it closed at 10,024.37. On Friday, the index lost 192.48 points, or 1.66 percent.
It was the Nikkei's sixth straight day of decline and its single largest one-day drop since May 10, 2004, when the index lost 554.12 points.
Stocks plunged because of concerns about the spreading anti-Japan protests in China, as well as Wall Street's steep declines last week.
Investors are growing nervous about the U.S. stock outlook and China-Japan tensions, Shinko Securities equity strategist Tsuyoshi Segawa said.
Even when the Nikkei lost 14 percent between April and May last year, "situations surrounding the market . . . weren't as uncertain as they are now," Segawa told Dow Jones Newswires.
Japan's top government spokesman voiced worry about the plunge.