Matrix elements of irreducible representations of the Lorentz group are
calculated on the basis of complex angular momentum. It is shown that
Laplace-Beltrami operators, defined in this basis, give rise to Fuchsian
differential equations. An explicit form of the matrix elements of the Lorentz
group has been found via the addition theorem for generalized spherical
functions. Different expressions of the matrix elements are given in terms of
hypergeometric functions both for finite-dimensional and unitary
representations of the principal and supplementary series of the Lorentz group.