  The notion of a multiplier of a group X is generalized to that of a
C*-multiplier by allowing it to have values in an arbitrary C*-algebra A. On
the other hand, the notion of the action of X in A is generalized to that of a
projective action of X as linear transformations of the space of continuous
functions with compact support in X and with values in A. It is shown that
there exists a one-to-one correspondence between C*-multipliers and projective
actions. C*-multipliers have been used to define twisted group algebras. On the
other hand, the projective action tau can be used to construct the crossed
product algebra A x_tau X. Both constructions are unified in the present
approach. The results are applicable in mathematical physics. The multiplier
algebra of the crossed product algebra A x_tau X contains Weyl operators
{W(x),x in X}. They satisfy canonical commutation relations w.r.t. the
C*-multiplier. Quantum spacetime is discussed as an example.
