  We study the time evolution of wave packets at the mobility edge of
disordered non-interacting electrons in two and three spatial dimensions. The
results of numerical calculations are found to agree with the predictions of
scaling theory. In particular, we find that the $k$-th moment of the
probability density $<r^k >(t)$ scales like $t^{k/d}$ in $d$ dimensions. The
return probability $P(r=0,t)$ scales like $t^{-D_2/d}$, with the generalized
dimension of the participation ratio $D_2$. For long times and short distances
the probability density of the wave packet shows power law scaling
$P(r,t)\propto t^{-D_2/d}r^{D_2-d}$. The numerical calculations were performed
on network models defined by a unitary time evolution operator providing an
efficient model for the study of the wave packet dynamics.
