We present real-time studies of nucleation and growth of 180° ferroelectric domains in congruent LiTaO3under an external electric field using optical microscopy. The measured wall velocities are an order of magnitude higher than those reported before using ex-situ techniques. Merger of growing domains form serrated fronts which show a futher order of magnitude increase in wall velocity. A time dependent velocity is required in the Avrami theory to explain the change in domain area versus time at a constant electric field. We believe that these features of domain switching must be considered when studying ferroelectrics in general.