A nonlinear non-affine multichannel system under the influence of external unmatched disturbances is considered as a control plant. Disturbances are assumed to be repeatedly differentiable unknowns, but limited functions of time. The tracking problem of the specified trajectories by output variables is posed under the assumption that the elements of the relative degree vector are the same and the plant model is reducible to a square block-canonical input-output form. Based on this form, a block synthesis of the tracking system with the formation of linear local feedbacks is performed. Assuming that only output variables and reference signals are measured, dynamic generators of external and reference actions are absent, variants of constructing an observer of mixed variables and generalized disturbances (functions of the state variables of the original system, external actions and their derivatives) are considered for information support of the tracking system. A new approach to designing an observer with piecewise linear corrective actions is proposed, combining the principles of constructing an observer of a standard structure and a dynamic differentiator, which is distinguished by a simpler adjustment of gain factors compared to typical solutions. Inequalities are formalized, on the basis of which the correction coefficients are selected independently of each other (in contrast to the observer of the standard structure). It is shown that the tracking system with closed linear feedbacks is a convenient basis for estimating mixed variables (in contrast to the usual canonical form and standard dynamic differentiators).