The newly discovered three-dimensional strong topological insulators (STIs) exhibit topologically protected Dirac surface states(1,2). Although the STI surface state has been studied spectroscopically, for example, by photoemission(3-5) and scanned probes(6-10), transport experiments(11-17) have failed to demonstrate the most fundamental signature of the STI: ambipolar metallic electronic transport in the topological surface of an insulating bulk. Here we show that the surfaces of thin (similar to 10 nm), low-doped Bi2Se3 (approximate to 10(17) cm(-3)) crystals are strongly electrostatically coupled, and a gate electrode can completely remove bulk charge carriers and bring both surfaces through the Dirac point simultaneously. We observe clear surface band conduction with a linear Hall resistivity and a well-defined ambipolar field effect, as well as a charge-inhomogeneous minimum conductivity region(18-20). A theory of charge disorder in a Dirac band(19-21) explains well both the magnitude and the variation with disorder strength of the minimum conductivity (2 to 5 e(2)/h per surface) and the residual (puddle) carrier density (0.4 x 10(12) to 4 x 10(12) cm(-2)). From the measured carrier mobilities 320-1,500 cm(2) V-1 s(-1), the charged impurity densities 0.5 x 10(13) to 2.3 x 10(13) cm(-2) are inferred. They are of a similar magnitude to the measured doping levels at zero gate voltage (1 x 10(13) to 3 x 10(13) cm(-2)), identifying dopants as the charged impurities.