TOKYO — Japan’s Nikkei 225 share index plunged as much as 10% on Monday on heavy selling triggered by worries the U.S. economy may be in worse shape than had been expected. The Nikkei index was down more than 3,500 points at 32,385.01 by midafternoon Monday in Tokyo. It dropped 5.8% on Friday and it is headed for its worst two-day decline ever. The Nikkei’s biggest single-day rout was a plunge of 3,836 points, or 14.9%, on the day dubbed “Black Monday” in October 1987. Share prices have fallen in Tokyo since the Bank of Japan raised its benchmark interest rate on Wednesday. The benchmark is now at about the level it was a year ago.