The space and time evolution of the internal optical field intensity inside weakly absorbing micron-sized particles at diffraction of femtosecond laser radiation is studied numerically using the Fourier method in combination with the Mie theory. Peculiarities are found in the dynamics of formation of the spatial distribution of the femtosecond pulse intensity in particles at various pulse duration, laser beam shape, particle size, and illumination geometry. It is shown that at nonstationary diffraction we almost always observe resonant excitation of the internal optical field, at which the natural frequencies of one or several high-Q resonance modes of the particle fall in the central part of the spectrum of the incident pulse. This leads to the delay of the radiation in the particle and to the decrease of the temporally absolute peak of the internal field intensity as compare dto the stationary case. The decrease of the peak intenstity is the greatest in the case of exact resonance, when it may achieve several orders of magnitude. Illumination of the particle by a narrow Gaussian beam of femtosecond duration directed at the particle center gives a gain in the internal field intensity as compared to the edge incidence of the radiation.