[en] Fluctuations of the particle densities in semi-central collisions of ¹⁶O, ²⁸Si and ³²S nuclei with heavy components of nuclear emulsion at different primary energies are studied. Various methods of data analysis have been used, ranging from the examination of the structure in individual events, to the study of the average factorial moments and conventional correlation functions. Emphasis is put on the analysis of the average factorial moments which appear to be the most efficient tool in searching for dynamical fluctuations. Unquestionable evidence for non-statistical fluctuations is found in ¹⁶O and ³²S interactions with Ag/Br targets at 200 GeV/nucleon. Specifically, I observe larger deviations of individual pseudorapidity distributions from a smooth spectrum and more frequent occurrence of narrow spikes than expected from Monte Carlo simulations based on independent emission of secondary particles. Clear intermittency signals are found in pseudorapidity and azimuthal angle distributions. Intermittency effect is also observed in two dimensions, where it has turned out to be stronger and cleaner than in one-dimensional distributions. Multiplicity distributions in the restricted phase space domains are well described by the negative binomial formula and consequently I find that factorial moments can be equivalently obtained from the parameters of the negative binomial fits. On the other hand investigation of the two-particle correlation functions in pseudorapidity provides no supporting evidence for pronounced non-statistical correlations. The occurrence of large particle-density fluctuations is expected in the presence of phase transition in ultrarelativistic nuclear collisions. Even though the obtained results can not be unambigously interpreted as an indication for quark-gluon plasma formation, nevertheless they clearly show that some collective effects are present in high energy nucleus-nucleus collisions. 84 refs. (author)