Hong Kong shares tumbled down on Thursday, tracking regional and US markets, as investors took profits following a rally over the two previous days. The Hang Seng index fell 2.6 percent, to 21,562.50, while the China Enterprises Index lost 2.0 percent, to 9,780.16 points. On Wednesday, the Hang Seng jumped 4.1 percent in its best one-day performance in nearly four years. Asian markets retreated on Thursday, following a correction on Wall Street. Japan's Nikkei fell 2.5 percent. All main sectors fell, with a key index tracking energy shares slumping 4.2 percent.