  We have obtained multi-fibre intermediate-resolution optical spectroscopy of
94 photometric and proper motion selected low-mass star and brown dwarf
candidates in Upper Sco with AAT/AAOmega. We have estimated the spectral types
and measured the equivalent widths of youth and gravity diagnostic features to
confirm the spectroscopic membership of about 95% of the candidates extracted
from 6.5 square degrees in Upper Sco. We also detect lithium in the spectra
with the highest signal-to-noise, consolidating our conclusions about their
youth. Furthermore, we derive an estimate of our selections using spectroscopic
data obtained for a large number of stars falling into the instrument's
field-of-view. We have estimated the effective temperatures and masses for each
new spectroscopic member using the latest evolutionary models available for
low-mass stars and brown dwarfs. Combining the current optical spectroscopy
presented here with near-infrared spectroscopy obtained for the faintest
photometric candidates, we confirm the shape and slope of our earlier
photometric mass function. The luminosity function drawn from the spectroscopic
sample of 113 USco members peaks at around M6 and is flat at later spectral
type. We may detect the presence of the M7/M8 gap in the luminosity function as
a result of the dust properties in substellar atmospheres. The mass function
may peak at 0.2 Msun and is quite flat in the substellar regime. We observe a
possible excess of cool low-mass brown dwarfs compared to IC 348 and the
extrapolation of the field mass functions, supporting the original hypothesis
that Upper Sco may possess an excess of brown dwarfs. This result shows that
the selection of photometric candidates based on five band photometry available
from the UKIDSS GCS and complemented partially by proper motions can lead to a
good representation of the spectroscopic mass function (abridged).
