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In this paper (paper I) we present optical long-slit spectroscopy and imagingof the protoplanetary nebula CRL618. The optical lobes of CRL618 consist ofshock-excited gas, which emits many recombination and forbidden lines, anddust, which scatters light from the innermost regions. From the analysis of thescattered Halpha emission, we derive a nebular inclination of i=24+-6 deg. Thespectrum of the innermost part of the east lobe (visible as a bright, compactnebulosity close to the star in the Halpha HST image) is remarkably differentfrom that of the shocked lobes but similar to that of the inner HII region,suggesting that this region represents the outermost parts of the latter. Wefind a non-linear radial variation of the gas velocity along the lobes.  The largest projected LSR velocities (~80 km/s) are measured at the tips ofthe lobes, where the direct images show the presence of compact bow-shapedstructures. The velocity of the shocks in CRL618 is in the range ~75-200 km/s,as derived from diagnostic line ratios and line profiles. We report abrightening (weakening) of [OIII]5007AA ([OI]6300AA) over the last ~10 yearsthat may indicate a recent increase in the speed of the exciting shocks. Fromthe analysis of the spatial variation of the nebular extinction, we find alarge density contrast between the material inside the lobes and beyond them:the optical lobes seem to be `cavities' excavated in the AGB envelope byinteraction with a more tenuous post-AGB wind. The electron density, with amean value n_e~5E3-1E4 cm-3, shows significant fluctuations but no systematicdecrease along the lobes, in agreement with most line emission arising in athin shell of shocked material (the lobe walls) rather than in the post-AGBwind filling the interior of the lobes. (...)Comment: 21 pages, 10 figures, accepted for publication in Ap

Physical Structure of the Proto–Planetary Nebula CRL 618. I. Optical Long‐Slit Spectroscopy and Imaging