  An action principle is presented for Vasiliev's Bosonic higher spin gauge
theory in four spacetime dimensions. The action is of the form of a broken
topological field theory, and arises by an extension of the MacDowell-Mansouri
formulation of general relativity. In the latter theory the local degrees of
freedom of general relativity arise by breaking the gauge invariance of a
topological theory from $sp(4)$ to the Lorentz algebra. In Vasiliev's theory
the infinite number of degrees of freedom with higher spins similarly arise by
the breaking of a topological theory with an infinite dimensional gauge
symmetry extending $sp(4)$ to the Lorentz algebra.
  The Hamiltonian formulation of Vasilev's theory is then derived from our
action, and it is shown that the Hamiltonian is a linear combination of
constraints, as expected for a diffeomorphism invariant theory. The constraint
algebra is computed and found to be first class.
