  Detailed magnetization measurements in La_{5/8-y}Pr_{y}Ca_{3/8} MnO_{3},
including magnetic relaxation properties, demonstrate the dynamic nature of the
phase separated state in manganites. The difference between the
field-cooled-cooling and zero-field-cooled magnetization curves signals the
existence in the latter of blocked metastable states separated by high energy
barriers. Results of the magnetic viscosity show that the system becomes
unblocked in a certain temperature window, where large relaxation rates are
observed. We propose a simple phenomenological model in which the system
evolves through a hierarchy of energy barriers, which separates the coexisting
phases. The calculated magnetization curves using this model reproduce all the
qualitative features of the experimental data. The overall results allowed us
to construct an H-T phase diagram, where frozen and dynamical phase separation
regions are clearly distinguished.
