Constitutive equations are derived for the viscoelastic response of filled elastomers at finite strains. A particle-reinforced rubber is thought of as a composite where regions with low concentrations of junctions between chains are randomly distributed in the bulk material. The onset of these inclusions is associated with an inhomogeneity in the spatial distribution of a cross-linker and a filler during the mixing process. With reference to the theory of transient networks, the time-dependent behavior of an elastomer is modelled as thermally activated processes of breakage and reformation of strands in domains with low concentrations of junctions, whereas junctions in the bulk medium are treated as permanent. Stress-strain relations are developed by using the laws of thermodynamics. Adjustable parameters in the constitutive equations are found by fitting experimental data in tensile relaxation tests for several grades of unfilled and carbon black filled natural rubber. It is demonstrated that (i) the average relaxation time noticeably grows with elongation ratio, which is explained by mechanically-induced crystallization of macromolecules, and (ii) the relaxation spectrum of a filled elastomer is not affected by mechanical pre-treatment and thermal recovery at elevated temperatures.