We report Keck high-resolution echelle spectroscopic observations of two very cool field dwarfs discovered by the near-IR photometric survey DENIS. DENIS-P J1228.2-1547 shows a conspicuous LiI resonance line that confirms it as an incontrovertible brown dwarf (BD). From the presence of Li, and its low surface temperature, we estimate from theoretical models a mass and an age upper limit of ~ 60 jupiters and ~ 10(9) years. The other DENIS object shows no detectable LiI line in our data, and thus we infer a mass >=60 jupiters for it. It could be a high-mass BD or very low-mass star. Both objects have modest radial velocities that suggest they are kinematically young. They show the strongest and broadest resonance line profiles from low ionization species ever seen. The extreme breadth of the KI lines are good further indicators of the expected high gravity in very low-mass dwarfs. We suggest a new spectral class, ``L'', for objects cooler than M-type (as these are) that do not show TiO molecular bands. The confirmation of at least one field BD in only ~ 1% of the final DENIS survey is a strong indication of the presence of a numerous population of these objects in the solar neighborhood. Based on observations obtained at the W.M. Keck Observatory, which is operated jointly by the University of California and the Californian Institute of Technology.