In this work we considered photodissociation dynamics of an ensemble of diatomic molecules under strong coupling conditions, using a semiclassical model where the electromagnetic field is described by the Maxwell equations and the molecular system is treated using a self-consistent mean field approximation. This yields a set of coupled Maxwell-Schrödinger equations with the molecular ro-vibrational degrees of freedom explicitly included. The combined effect of collective molecular excitation (that is manifested in a considerable polariton energy shift) and the individual nature of molecular nuclear dynamics (namely, nuclear motion responds to the excitation state of individual molecules) is shown to have significant consequences for the dissociation dynamics following polariton excitation. Specifically, the rate of nuclear separation following polariton formation is initially slower than the corresponding process following excitations into spectral regimes where a polariton is not formed. A rationalization of the observed effect proposed was based on the recognition that nuclear separation in individual (excited) molecules is essentially a dephasing process that tends to destroy the polariton and therefore has an energy cost. The resulting local activation barrier leads to a local (therefore transient slow-down of the nuclear separation dynamics. Further consequences of the conflicting effects of collective excitation and individual molecular motions should be expressed in the presence of thermal interactions and will be explored in the future. ) and ( )). Franck-Condon factors, , as defined in Eq. ( ´