00-15E

The procedure of reducing canonical field degrees of freedom for a system of charged particles plus field in the constrained Hamiltonian formalism is elaborated up to the first order in the coupling constant expansion. The canonical realization of the Poincar\'e algebra in the terms of particle variables is found. The relation between covariant and physical particle variables in the Hamiltonian description is written. The system of particles interacting by means of scalar and vector massive fields is also considered. The first order approximation in $c^{-2}$ is examined. An application to the calculating the relativistic partition function of interacting particle system is discussed.