This is the first of a series of three papers exploring the connection between the multiwavelength properties of AGNs in nearby early-type galaxies and the characteristics of their hosts. We selected two samples, both with high resolution 5 GHz VLA observations available and providing measurements down to 1 mJy level, reaching radio-luminosities as low as 10^19 W/Hz. We focus on the 116 radio-detected galaxies as to boost the fraction of AGN with respect to a purely optically selected sample. Here we present the analysis of the optical brightness profiles based on archival HST images, available for 65 objects. We separate early-type galaxies on the basis of the slope of their nuclear brightness profiles, into core and power-law galaxies following the Nukers scheme, rather than on the traditional morphological classification (i.e. into E and S0 galaxies). Our sample of AGN candidates is indistinguishable, when their brightness profiles are concerned, from galaxies of similar optical luminosity but hosting weaker (or no) radio-sources. We confirm previous findings that relatively bright radio-sources (L_r > 10^(21.5) W/Hz) are uniquely associated to core galaxies. However, below this threshold in radio-luminosity core and power-law galaxies coexist and they do not show any apparent difference in their radio-properties. Not surprisingly, since our sample is deliberately biased to favour the inclusion of active galaxies, we found a higher fraction of optically nucleated galaxies. Addressing the multiwavelength properties of these nuclei will be the aim of the two forthcoming papers.
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