We have used the Nordic Optical and the William Herschel Telescopes to obtain images and long-slit spectra of Cygnus A under conditions of sub-arcsecond seeing. The high spatial resolution and accurate positional information obtained from a comparison between the images and spectra allow us to study in detail the kinematics and excitation of the extended ionized gas. Strong velocity gradients and complex line profiles are detected in positions close to the radio axis corresponding to the emission line clouds recently resolved by HST. The results are interpreted in terms of a strong interaction between the radio jet and the interstellar medium.