The advances in hardware technology enable the inclusion of an SMP node into a cluster of PC or even clusters of SMP. These are becoming viable alternatives for high performance computing. The challenge is the exploration of the computational resources that these hardware platforms provide. A hybrid programming paradigm which uses a shared memory architecture through multi threading and through a message passing model for inter node communication is an alternative. However, programming in such a paradigm is very hard. This work presents CPAR-Cluster, a runtime system that provides shared memory abstraction on top of a cluster composed by mono and multiprocessor nodes. Its implementation is at the library level and does not require special resources such as specific hardware or operating system modifications. Models, strategies, implementation aspects and some results are presented.