  Classical relativistic system of point particles coupled with an
electromagnetic field is considered in the three-dimensional representation.
The gauge freedom connected with the chronometrical invariance of the
four-dimensional description is reduced by use of the geometrical concept of
the forms of relativistic dynamics. The remainder gauge degrees of freedom of
the electromagnetic potential are analysed within the framework of Dirac's
constrained Hamiltonian mechanics in the front form of dynamics. The results
are implemented to the problems of relativistic statistical mechanics. Based on
the corresponding Liouville equation the classical partition function of the
system is written down in a gauge-invariant manner and an integration over
field variables is performed.
