The arguments for the possibility of violation of P and CP symmetries of strong interactions at finite temperature are presented. A new way of observing these effects in heavy ion collisions is proposed -- it is shown that parity violation should manifest itself in the asymmetry between positive and negative pions with respect to the reaction plane. Basing on topological considerations, we derive a lower bound on the magnitude of the expected asymmetry, which may appear within the reach of the current and/or future heavy ion experiments.