We present the results of photometric and spectroscopic follow-ups of the lowest mass member candidates in the nearest OB association, Upper Scorpius (5-10 Myr; 145+/-17 pc), with the Gran Telescopio de Canarias (GTC) and European Southern Observatory (ESO) Very Large Telescope (VLT). We confirm the membership of the large majority (>80%) of the candidates selected originally photometrically and astrometrically based on their spectroscopic features, weak equivalent widths of gravity-sensitive doublets, and radial velocities. Confirmed members follow a sequence over a wide magnitude range (J=17.0-19.3 mag) in several colour-magnitude diagrams with optical, near-, and mid-infrared photometry, and have near-infrared spectral types in the L1-L7 interval with likely masses below 15 Jupiter masses. We find that optical spectral types tend to be earlier than near-infrared spectral types by a few subclasses for spectral types later than M9. We investigate the behaviour of spectral indices defined in the literature as a function of spectral type and gravity by comparison with values reported in the literature for young and old dwarfs. We also derive effective temperatures in the 1900-1600K from fits of synthetic model-atmosphere spectra to the observed photometry but we caution the procedure carries large uncertainties. We determine bolometric corrections for young L dwarfs with ages of ~5-10 Myr (Upper Sco association) and find them similar in the J-band but larger by 0.1-0.4 mag in the K-band with respect to field L dwarfs. Finally, we discovered two faint young L dwarfs, VISTAJ1607-2146 (L4.5) and VISTAJ1611-2215 (L5) that have H$alpha$ emission and possible flux excesses at 4.5 microns, pointing towards the presence of accretion from a disk onto the central objects of mass below ~15 Jupiter masses at the age of 5-10 Myr.
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