  We report on the observation of ultralong range interactions in a gas of cold
Rubidium Rydberg atoms. The van-der-Waals interaction between a pair of Rydberg
atoms separated as far as 100,000 Bohr radii features two important effects:
Spectral broadening of the resonance lines and suppression of excitation with
increasing density. The density dependence of these effects is investigated in
detail for the S- and P- Rydberg states with main quantum numbers n ~ 60 and n
~ 80 excited by narrow-band continuous-wave laser light. The density-dependent
suppression of excitation can be interpreted as the onset of an
interaction-induced local blockade.
