We present a report on the strong X5.3 solar flare which occurred on 25 August 2001, producing high-level gamma-ray activity, nuclear lines and a dramatic long-duration white-light continuum. The bulk of millimeter radio fluxes reached a peak of similar to100 000 solar flux units at 89.4 GHz, and a few thousands of solar flux units were detected in the submillimeter range during the impulsive phase. In this paper we focus on and discuss (i) the implications inferred from high frequency radio observations during the impulsive phase; (ii) the dynamics of the low corona active region during the impulsive phase. In particular we found that 4-5 x 10(36) accelerated (>20 keV) electrons s(-1) radiating in a 1000-1100 G region, are needed to explain the millimeter to submillimeter-wave emissions. We present evidence that the magnetic field in the active region was very dynamic, and that strong non-thermal processes were triggered by the appearance of new, compact, low-lying (few thousand kilometers) loop systems, suggesting the acceleration site(s) were also located in the low solar atmosphere.2231181199