  (Abridged) The open cluster NGC 1893, illuminating the HII region IC410,
contains a moderately large population of O-type stars and is one of the
youngest clusters observable in the optical range. We have probed the stellar
population of NGC 1893 in an attempt to determine its size and extent. We
classify a large sample of cluster members with new intermediate resolution
spectroscopy. We use H-alpha slitless spectroscopy of the field to search for
emission line objects, identifying 18 emission-line PMS stars. We then combine
existing optical photometry with 2MASS JHKs photometry to detect stars with
infrared excesses, finding close to 20 more PMS candidates. While almost all
stars earlier than B2 indicate standard reddening, all later cluster members
show strong deviations from a standard reddening law, which we interpret in
terms of infrared excess emission. Emission-line stars and IR-excess objects
show the same spatial distribution, concentrating around two localised areas,
the immediate vicinity of the pennant nebulae Sim 129 and Sim 130 and the area
close to the cluster core where the rim of the molecular cloud associated with
IC 410 is illuminated by the nearby O-type stars. In and around the emission
nebula Sim 130 we find three Herbig Be stars with spectral types in the B1-4
range and several other fainter emission-line stars. We obtain a complete
census of B-type stars by combining Stroemgren, Johnson and 2MASS photometry
and find a deficit of intermediate mass stars compared to massive stars. We
observe a relatively extended halo of massive stars surrounding the cluster
without an accompanying population of intermediate-mass stars. The overall
picture of star formation in NGC 1893 suggests a very complex process.
