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Aims: We study the optical spectral properties of a sample of stars showingfar infrared colours similar to those of well-known planetary nebulae. Thelarge majority of them were unidentified sources or poorly known in theliterature at the time when this spectroscopic survey started, some 15 yearsago.  Methods: We present low-resolution optical spectroscopy, finding charts andimproved astrometric coordinates of a sample of 253 IRAS sources.  Results: We have identified 103 sources as post-AGB stars, 21 as ``transitionsources'', and 36 as planetary nebulae, some of them strongly reddened. Amongthe rest of sources in the sample, we were also able to identify 38 youngstellar objects, 5 peculiar stars, and 2 Seyfert galaxies. Up to 49 sources inour spectroscopic sample do not show any optical counterpart, and most of themare suggested to be heavily obscured post-AGB stars, rapidly evolving on theirway to becoming planetary nebulae.  Conclusions: An analysis of the galactic distribution of the sourcesidentified as evolved stars in the sample is presented together with a study ofthe distribution of these stars in the IRAS two-colour diagram. Finally, thespectral type distribution and other properties of the sources identified aspost-AGB in this spectroscopic survey are discussed in the framework of stellarevolution.Comment: 69 pages, 413 figures. Accepted by Astronomy and Astrophysic

A spectroscopic atlas of post-AGB stars and planetary nebulae selected from the IRAS point source catalogue