Abstract <p>Filamentation of high-power femtosecond pulses in a gas is of great theoretical and practical interest with relation to the study of large-scale spectral and temporal transformations of laser radiation in a medium and generation of extra-wide (supercontinuum) radiation. This radiation is of interest in nonlinear femtosecond diagnostics of environment, optical data transmission through an atmospheric channel, and modern optical material processing techniques. This paper experimentally studies the effect of gas medium (nitrogen) pressure in an optical cell on the characteristics of femtosecond laser radiation propagating under filamentation conditions. It is found that high nitrogen pressure (up to 11 atm) and sharp geometric focusing of femtosecond radiation result in its Kerr self-focusing, which transforms from the single filamentation mode into multiple post filamentation as the gas pressure increases. Due to the phase self-modulation of a femtosecond pulse and plasma generation in the gas, the radiation spectrum is significantly enriched and the pulse spectrum near-linearly broadens with an increase in the gas pressure in the cell. It is ascertained for the first time that the pulse spectrum asymmetrically broadens mainly to the long-wave region while the initial beam focusing sharpness is increasing. The average size of high-intensity post-filaments formed inside a beam decreases as the gas pressure increases in the optical cell and can be fractions of a millimeter.</p>