The Nikkei stock index briefly plunged more than 2 percent in early trading Thursday, weighed down by Wall Street declines overnight amid concern about the U.S. economy. In the first 15 minutes of trading, the 225-issue Nikkei Stock Average fell 556.28 points, or 1.59 percent, from Wednesday to 34,533.34. The broader Topix index was down 21.52 points, or 0.86 percent, at 2,467.69. On the top-tier Prime Market, decliners were led by nonferrous metal, wholesale trade and miscellaneous product shares.