This paper presents Discrete-Guided Diffusion (DGD), a novel framework that integrates discrete multi-agent pathfinding (MAPF) and a constrained generative diffusion model to solve the multi-robot motion planning (MRMP) problem. DGD decomposes the nonconvex MRMP problem into tractable subproblems, combines discrete MAPF solutions with constraint optimization techniques to capture complex spatiotemporal dependencies, and incorporates a lightweight constraint recovery mechanism to ensure path feasibility. This approach demonstrates state-of-the-art performance, achieving planning efficiency and high success rates while scaling up to 100 robots in large, complex environments.