A wireless communication system is overloaded when the number of transmitted signals exceeds the number of receive antennas. The presence of the resulting cochannel interference (CCI) under overload causes linear detection techniques to perform poorly. We develop a unified approach to the separation and detection of the user signals for an overloaded system using a novel iterative list-based multiuser detector. It combines a linear preprocessor with a nonlinear list detector and approximates optimum joint maximum-likelihood detection at lower complexity. Complexity savings are achieved by first, exploiting the spatial separation of the users to mitigate CCI in the preprocessor stage and second, by estimating residual CCI in the following list detection stage. The proposed list detection algorithm is applied to receivers with either a uniform circular array or a uniform linear array. The preprocessor is implemented using either a special purpose spatial filter to mitigate the CCI or maximum ratio diversity combining to achieve diversity gain. Simulation results and a complexity analysis indicate that the approach is suitable for practical application.