I consider a dynamics of an ensemble of classical nonlinear oscillators with dissipative and reactive coupling. It is shown that a state with an incoherent initial phase distribution can be unstable with respect to a small coherent initial component. The mathematical approach is based on a representation of the system as a set of collective modes with different total phase shift between the oscillators. The nonlinear stage of the process is analyzed to show that in a dissipative system, a coherent impulse is generated, and in a Hamiltonian system, a periodic set of short coherent pulses is generated. Physical examples of such a "coherentization" ("classical maser effect") include possible electromagnetic and acoustic systems. Historically, similar processes were used in powerful electronic generators: cyclotron resonance masers, or gyrotrons.