In this paper, the authors consider an optimal control problem subject to a semilinear elliptic state equation with non-differentiable cost functional. They derive first and second order necessary and sufficient optimality conditions. From the first order optimality conditions, the authors deduce extra regularity for the optimal control which is essential in deriving error estimates. They show a-priori finite element error estimates for piecewise constant discretization of the control and piecewise linear discretization of the state. Error estimates for the variational discretization of the problem are also obtained. Finally, numerical results confirm the convergence rates.