  We report new nearby L and late-M dwarfs (d_phot <= 30 pc) discovered in our
search for nearby ultracool dwarfs (I-J >= 3.0, later than M8.0) at low
Galactic latitude (|b| < 15 degr) over 4,800 square degrees in the DENIS
database. We used late-M (>=M8.0), L, and T dwarfs with accurate trigonometric
parallaxes to calibrate the M_J versus I-J colour-luminosity relation. The
resulting photometric distances have standard errors of ~15%, which we used to
select candidates d_phot <= 30 pc. We measured proper motions from multi-epoch
images found in the public archives ALADIN, DSS, 2MASS, DENIS, with at least
three distinct epochs and time baselines of 10 to 21 years. We then used a
Maximum Reduced Proper Motion cutoff to select 28 candidates as ultracool
dwarfs (M8.0--L8.0) and to reject one as a distant red star. No T dwarf
candidates were found in this search which required an object to be detected in
all three DENIS bands. Our low-resolution optical spectra confirmed that 26 of
them were indeed ultracool dwarfs, with spectral types from M8.0 to L5.5. Two
contaminants and one rejected by the Maximum Reduced Proper Motion cutoff were
all reddened F-K main sequence stars. 20 of these 26 ultracool dwarfs are new
nearby ultracool dwarf members, three L dwarfs within 15 pc with one L3.5 at
only ~10 pc. We determine a stellar density of \bar{\Phi}_J cor=(1.64 +-
0.46).10^{-3} dwarfs pc^{-3} mag^{-1} over 11.1 <= M_J <= 13.1 based on that
sample of M8--L3.5 ultracool dwarfs. Our ultracool dwarf density value is in
good agreement with the Cruz et al. measurement of the ultracool dwarf density
at high Galactic latitude.
