  We use numerical simulations to predict peculiar magnetotransport
fingerprints in polycrystalline graphene, driven by the presence of grain
boundaries of varying size and orientation. The formation of Landau levels is
shown to be restricted by the polycrystalline morphology, requiring the
magnetic length to be smaller than the average grain radius. The nature of
localization is also found to be unusual, with strongly localized states at the
center of Landau levels (including the usually highly robust zero-energy state)
and extended electronic states lying between Landau levels. These extended
states percolate along the network of grain boundaries, resulting in a finite
value for the bulk dissipative conductivity and suppression of the quantized
Hall conductance. Such breakdown of the quantum Hall regime provoked by
extended structural defects is also illustrated through two-terminal
Landauer-B\"uttiker conductance calculations, indicating how a single grain
boundary induces cross-linking between edge states lying at opposite sides of a
ribbon geometry.
