  This paper is a study of the Lie groups of point symmetries admitted by a
system describing a non-stationary planar flow of an ideal plastic material.
For several types of forces involved in the system, the infinitesimal
generators which generate the Lie algebra of symmetries have been obtained. In
the case of a monogenic force, the classification of one- and two- dimensional
subalgebras into conjugacy classes under the action of the group of
automorphisms has been accomplished. The method of symmetry reduction is
applied for certain subalgebra classes in order to obtain invariant solutions.
