  We give a simple proof of the fact that for a large class of quasilinear
elliptic equations and systems the solutions that minimize the corresponding
energy in the set of all solutions are radially symmetric. We require just
continuous nonlinearities and no cooperative conditions for systems. Thus, in
particular, our results cannot be obtained by using the moving planes method.
In the case of scalar equations, we also prove that any least energy solution
has a constant sign and is monotone with respect to the radial variable.
