This paper deals with the design of a hierarchical control scheme for complex islanded Alternate Current (AC) microgrids. The proposed solution relies on the combined use of Model Predictive Control (MPC) and Sliding Mode Control (SMC). The model of the microgrid includes several Distributed Generation Units (DGus), affected by unknown load dynamics and modelling uncertainties. Moreover, they are connected according to an arbitrary complex and meshed topology, taking into account the interconnecting line dynamics. The proposed control scheme consists of two control loops. A centralized MPC supervisor generates the voltage reference values for each DGu, while fulfilling input and state constraints on the basis of a reduced order model of the plant. A Suboptimal Second Order Sliding Mode (SSOSM) control is locally designed for each DGu to track, in a decentralized way, the voltage references generated by the supervisor. Simulation results confirm the effectiveness of the proposed control scheme.