In this paper, a new algorithm for cooperative path planning in a multi-robot system is introduced. The algorithm is specially designed for a distributed system with a large number of robots, all of which should be able to reach their respective goal positions without blocking each other, even if the environment is heavily constricted by obstacles. The basic idea of the approach is based on fully distributed path planning without any central instance, but with the ability of communication and cooperation between the robots. Path planning is done on local sections of the time-space configuration space. Dynamic conflicts between the robots are solved by the heuristic adjustment of priority values. By a continuous enhancement of all plans the algorithm is very robust against dynamic changes and erroneous robot behavior.