  We analyze the symmetry changes of the paramagnetic to the A-type
antiferromagnetic and to the ferromagnetic phase transitions in undoped and
moderately doped LaMnO$_3$, respectively. We show that in the
orthorhombic-distorted perovskite manganites the phase separation at low doping
is associated with the noncollinear nature of the magnetic orders permitted by
symmetry. A simple model for the competition between the two phase transitions
is put forward within the framework of Landau theory of phase transitions.
