Аннотация:A quasilinear system of hyperbolic equations describing plane one-dimensional relativistic oscillationsof electrons in a cold plasma is considered. For a simplified formulation, a criterion for the existence ofa global-in-time smooth solution is obtained. For the original system, a sufficient condition for singularityformation is found, and a sufficient condition for the smoothness of the solution within the nonrelativisticperiod of oscillations is established. In addition, it is shown that arbitrarily small perturbations of the trivialsolution lead to the formation of singularities in a finite time. The results can be used to construct and substantiatenumerical algorithms for modeling the breaking of plasma oscillations