  By means of molecular dynamics, we study a model system for colloidal
suspensions where the interaction is based on a competition between attraction
and repulsion. At low temperatures the relaxation time $\tau$ first increases
as a power law as a function of the volume fraction $\phi$ and then, due to the
finite lifetime of the bonded structures, it deviates from this critical
behavior. We show that colloidal gelation at low temperatures and low volume
fractions is crucially related to the formation of spanning long living
cluster. Besides agreeing with experimental findings in different colloidal
systems, our results shed new light on the different role played by the
formation of long living bonds and the crowding of the particles in colloidal
structural arrest.
