We present a study of the magnetic properties of the Pr 0.5Sr 0.5xCa xMnO 3 manganite (x=0.1 and 0.2) in the temperature region where phase separation occurs. This state is characterized by the presence of ferromagnetic (FM) inclusions inside an antiferromagnetic (AFM) matrix. The evolution of the magnetization (M) with magnetic field shows the existence of a critical field, H C, above which M rapidly increases, indicating a sudden expansion of the FM volume against the AFM one. We analyze this behavior and the response of the magnetic susceptibility at low fields (H<H C) in terms of a thermally activated motion of pinned FM/AFM elastic interfaces. The pinning mechanism is likely to be related to the martensitic accommodation strain around the magnetic and structural interfaces. From this analysis we estimate the size of the FM domains and the parameters that characterize the pinning potential.