A new scenario is proposed in which the continuous creation of stars results into a highly concentrated massive (globular cluster-like) stellar system. We show that under very general conditions a large-scale gravitational instability in the ISM that triggers the collapse of a massive cloud leads with the aid of newly-formed stars to a standing small radius cold and dense shell. After the initial evolution the shell begins to fragment while keeping its steady state location. This is due to a detailed balance established between the ram pressure from the collapsing cloud which together with the gravitational force excerted on the shell by the forming cluster acts against the mechanical energy deposited by the collection of new stars. We show that the shape of the mass spectrum of fragments is approximated at the high mass end by a power law with the slope -2.25 very close to the value that fits the universal IMF. Furthermore it presents a maximum near to one solar mass and a rapid change towards a much flatter slope for smaller fragments.