Organic-inorganic lead halide perovskites have recently received significant attention as active materials for high-performance photovoltaics and photo-detectors. However, the understanding of their operation mechanism remains limited. High-gain, low-voltage CH3NH3PbI3 photodetectors in various architectures are demonstrated herein. Photomultiplication in all structures with direct contact of fluorine-doped tin oxide (FTO) and perovskite with the highest responsivity 208 A W-1 corresponding to an incident photon-to-current efficiency of 47 000% is observed. Studying the dynamics and temperature dependence, a slow process with an activation energy of 420 +/- 90 meV in the time scale of seconds is found, which is essential to photocurrent multiplication. A model based on ion migration to explain the observed transients and the photomultiplication is developed. The accumulation of negative ionic charge at the FTO/perovskite interface under reverse bias lowers the FTO work function allowing for direct hole injection into the perovskite valence band. Under illumination, the conductivity of perovskite is increased and the device behaves similar to a photoconductor.