A method for computing minimum load shedding in a competitive electric power market is proposed. The minimum load-shedding problem is formulated as an optimal power flow (OPF), including equality and inequality nonlinear constraints which take into account the system's security conditions. A multiple predictor-corrector (MPC) higher order interior-point method was implemented and tested. The utilisation of an MPC method leads to both a smaller number of iterations for convergence and smaller computational times. The idea is to determine individual load-shedding percentages for each busbar to assure predefined security conditions. Several test power systems were used in the simulations, namely the IEEE 30, 57, 118 and 300 busbar systems. Also, two realistic power systems were used, a 464 busbar, corresponding to the interconnected Peruvian system, and a 2256 busbar corresponding to part (South-Southeast) of the interconnected Brazilian system. An analysis of the dual variables related to the optimisation problem is also carried out.