  We present and analyse HST/WFPC2 broad- and narrow-band observations of the
central regions of 19 nearby radio-loud early-type galaxies. Together with two
more galaxies they form a complete sample of Fanaroff & Riley Type I galaxies.
We obtained V- and I-band images and narrow-band images centered on the
\HalphaNII emission lines. We use archival data for six galaxies.
  We describe the data reduction, give isophotal fits and analyse the central
emission-line gas and dust distributions. Our main conclusions are the
following. Although obscuration by dust inhibits a direct determination of
central cusp slopes, the data suggest most but not all galaxies have shallow
cores. Dust is detected in all but two galaxies. There is a wide variety of
central dust morphologies, ranging from central disks to lanes and irregular
distributions. The analysis suggests the difference between disks and lanes is
intrinsic and not due to different viewing angles. Central emission-line gas is
detected in all galaxies. Extended low-surface-brightness emission is always
associated with the dust features. In a subsequent paper we will present a
detailed analysis of the relation between these central properties and the
nuclear activity.
