  We introduce the concept of soliton clusters -- multi-soliton bound states in
a homogeneous bulk optical medium, and reveal a key physical mechanism for
their stabilization associated with a staircase-like phase distribution that
induces a net angular momentum and leads to cluster rotation. The ringlike
soliton clusters provide a nontrivial generalization of the concepts of
two-soliton spiraling, optical vortex solitons, and necklace-type optical
beams.
