Phase synchronized entrainment of coupled oscillators with distributed natural frequencies is studied by exploring the dynamical manifestation. The route from partial to complete phase synchronization is identified as a cascade of transitions from high- to low-dimensional tori (quasiperiodicities) interrupted by intermittent chaos. Torus ON-OFF intermittency is found at the onset of desynchronization. Desynchronization-induced chaos originates from the mixing of intermittent ON-OFF timescales.