A Grid simulation infrastructure can facilitate a simulation comprising models of different grains and/or models of various types in nature to investigate large and complicated problems. On top of the infrastructure, a simulation of evacuating thousands of pedestrians in a large urban area has been constructed. A number of agent-based and computational models residing at two administrative domains operate together, which successfully presents the dynamics of the complex scenario at scales of both individual and crowd levels. Experimental results indicate that the proposed approach can effectively cope with the size and complexity of a scenario involving a large crowd.