The Nikkei stock index briefly fell more than 2 percent in early trading Friday after sell-offs in international markets overnight on fears over the adverse impact of monetary tightening by European central banks. In the first 15 minutes of trading, the 225-issue Nikkei Stock Average fell 519.81 points, or 1.97 percent, from Thursday to 25,911.39. The broader Topix index was down 36.23 points, or 1.94 percent, at 1,831.58. Every industry category on the top-tier Prime Market lost ground. Decliners were led by transportation equipment, metal product and electric appliance issues.