We have performed N-body simulations of the formation of intermediate mass black holes in young star clusters and the subsequent dynamical evolution of star clusters containing massive black holes. Our simulations show that runaway merging of massive main-sequence stars can produce an intermediate mass black hole in the M82 cluster MGG-11 if the initial concentration of this cluster is larger than a King W-0 = 9 model. This explains the detection of an ultra-luminous X-ray source in this cluster by Matsumoto et al. [Astrophys. J. 547 (2001), L25]. We also find that clusters with intermediate mass black holes evolve to have constant density cores in projection, which makes it unlikely that galactic core collapse globular clusters contain black holes.