  A discrete version of Lagrangian reduction is developed in the context of
discrete time Lagrangian systems on $G\times G$, where $G$ is a Lie group. We
consider the case when the Lagrange function is invariant with respect to the
action of an isotropy subgroup of a fixed element in the representation space
of $G$. In this context the reduction of the discrete Euler-Lagrange equations
is shown to lead to the so called discrete Euler-Poincar\'e equations. A
constrained variational principle is derived. The Legendre transformation of
the discrete Euler-Poincar\'e equations leads to discrete Hamiltonian
(Lie-Poisson) systems on a dual space to a semiproduct Lie algebra.
