This paper analyzes the symbol error probability (SEP) of cluster-based cooperative networks where a finite cluster of base stations jointly detect multiple in-cluster users in the presence of out- of-cluster interference. For such a network, we derive new accurate upper and lower bounds on the SEP of the in-cluster users with Rayleigh fading, arbitrary path loss, and M-ary phase-shift keying modulation. We further derive new asymptotic expressions to accurately characterize the SEP saturation limit arising from out-of-cluster interference. To obtain deeper insights, we extend our analysis to consider Rician fading with line- of-sight paths from the in-cluster users to their nearest base stations. Numerical examples illustrate the accuracy of our results and highlight novel aspects of fading channels, path loss, cluster configurations, and user locations on the SEP of cluster-based cooperative cellular networks.