The second harmonic of the laser light (2omega(0)) is observed on the rear side of thick solid targets irradiated by a laser beam at relativistic intensities. This emission is explained by the acceleration by the laser pulse in front of the target of short bunches of electrons separated by the period (or half the period) of the laser light. When reaching the rear side of the target, these electron bunches emit coherent transition radiation at 2omega(0). The observations indicate that, in our conditions, the minimum fraction of the laser energy transferred to these electron bunches is of the order of 1%.