  We show that in the supersymmetry framework described by a Poincar\'{e}
superalgebra with tensorial central charges the role of generalized
superconformal symmetry which contains all these central charges is played by
$OSp(1|2^{k})$, where k=3 for D=4. Following [1,2] we describe the free
supertwistor model for $OSp(1|8)$. It appears that in such a scheme the
tensorial central charges satisfy additional relations and the model describes
the tower of supersymmetric massless states with an arbitrary (integer and
half--integer) helicity spectrum.
