  Plasticity of two-dimensional discrete dislocation systems is studied. It is
shown, that at some threshold stress level the response becomes stress-rate
dependent. Below this stress level the stress-plastic strain relation exhibits
power-law type behavior. In this regime the plastic strain rate induced by a
constant external stress decays to zero as a power-law, which stems from the
scaling of the dislocation velocity distribution. The scaling is cut-off at a
time only dependent on the system size and the scaling exponent depends on the
external stress and on the initial correlations present in the system. These
results show, that the dislocation system is in a critical state everywhere we
studied below the threshold stress.
