Context: The connection of cluster mergers with the presence of extended, diffuse radio sources in galaxy clusters is still debated. An interesting case is the rich, merging cluster Abell 520, containing a radio halo. A recent gravitational analysis has shown in this cluster the presence of a massive dark core suggested to be a possible problem for the current cold dark matter paradigm. Aims: We aim to obtain new insights into the internal dynamics of Abell 520 analyzing velocities and positions of member galaxies. Methods: Our analysis is based on redshift data for 293 galaxies in the cluster field obtained combining new redshift data for 8 galaxies acquired at the TNG with data obtained by CNOC team and other few data from the literature. We also use new photometric data obtained at the INT telescope. We combine galaxy velocities and positions to select 167 cluster members around z ˜0.201. We analyze the cluster structure using the weighted gap analysis, the KMM method, the Dressler-Shectman statistics and the analysis of the velocity dispersion profiles. We compare our results with those from X-ray, radio and gravitational lensing analyses. Results: We compute a global line-of-sight (LOS) velocity dispersion of galaxies, σ v=1066-61+67 km s-1. We detect the presence of a high velocity group (HVG) with a rest-frame relative LOS velocity of { v_rf}˜ 2000 km s-1 with respect to the main system (MS). Using two alternative cluster models we estimate a mass range M(<1 h70-1 Mpc)=(4.0-9.6) × 1014 h70-1 M&sun;. We also find that the MS shows evidence of subclumps along two preferred directions. The main, complex structure NE 1+NE 2 (with a velocity comparable to that of the MS) and the SW structure (at { v_rf}˜ +1100 km s-1) define the NE-SW direction, the same of the merger suggested by X-ray and radio data. The E and W structures (at { v_rf}˜ -1150 and {v_rf}˜ - 300 km s-1) define the E-W direction. Moreover, we find no dynamical trace of an important structure around the lensing dark core. Rather, the HVG and a minor MS group, having different velocities, are roughly centered in the same position of the lensing dark core, i.e. are somewhat aligned with the LOS. Conclusions: We find that Abell 520 is definitely a very complex system. Our results suggest that we are looking at a cluster forming at the crossing of three filaments of the large scale structure. The filament aligned with the LOS and projected onto the center of the forming cluster might explain the apparent massive dark core shown by gravitational lensing analysis.