The growing demand for computational power causes that Grids are becoming mission-critical components in research and industry, offering sophisticated solutions in leveraging large-scale computing and storage resources. The nature a Grid in which resources are usually shared among multiple organizations offering resources under their control based on the “best effort” approach with no guarantee concerning the quality-of-service may be inadequate to support large-scale simulations. Requirements of such simulations often exceed capabilities of a single computing center causing the need to simultaneously allocate and synchronize resources belonging to many administrative domains whose functionality is missing in leading grid middlewares preventing researchers from executing large-scale simulations in grids. The paper presents tools and services that were designed to build multilayered infrastructure capable of dealing with computationally intensive large-scale simulations in the grid environment. The developed and deployed middleware enables computing clusters in different administrative domains to be virtually welded into a single powerful compute resource that can be treated as a quasi-opportunistic supercomputer. We describe the middleware developed in the QosCosGrid project and being enhanced under the PL-Grid national grid initiative, which provides advance reservation and resource co-allocation functionality as well as support for parallel large-scale applications based on OpenMPI (for C/C++ and Fortran) or ProActive for Java.