This article describes an observation of mixing-induced CP violation and a measurement of the CP violation parameter, sin2φ1, with the Belle detector at the KEKB asymmetric e+e− collider. Using a data sample of 29.1 fb−1 recorded on the Υ(4S) resonance that contains 31.3 million BB¯ pairs, we reconstruct decays of neutral B mesons to the following CP eigenstates: J/ψK0S, ψ(2S)K0S, χc1K0S, ηcK0S, J/ψK0L and J/ψK*0. The flavor of the accompanying B meson is identified by combining information from primary and secondary leptons, K± mesons, Λ baryons, slow and fast pions. The proper-time interval between the two B meson decays is determined from the distance between the two decay vertices measured with a silicon vertex detector. The result sin2φ1=0.99±0.14(stat)±0.06(syst) is obtained by applying a maximum likelihood fit to the 1137 candidate events. We conclude that there is large CP violation in the neutral B meson system. A zero value for sin2φ1 is ruled out by more than six standard deviations.