Nuclear star clusters (NSCs) are the densest stellar systems in the Universe.They can be found at the center of all galaxy types, but tend to favor galaxiesof intermediate stellar mass around 10$^9\,$M$_{\odot}$[1, 2]. Currently, twomain processes are under debate to explain their formation: in-situstar-formation from gas infall[3] and migration and merging of globularclusters (GCs) caused by dynamical friction[4]. Studies[5-9] of NSC stellarpopulations suggest that the former predominates in massive galaxies, thelatter prevails in dwarf galaxies, and both contribute equally at intermediatemass. However, up to now, no ongoing merger of GCs has yet been observed toconfirm this scenario. Here we report the serendipitous discovery of five dwarfgalaxies with complex nuclear regions, characterized by multiple nuclei andtidal tails, using high resolution images from the Hubble Space Telescope.These structures have been reproduced in complementary N-body simulations,supporting the interpretation that they result from migrating and merging ofstar clusters. The small detection rate and short simulated timescales (below100 Myr) of this process may explain why this has not been observed previously.This study highlights the need of large surveys with high resolution to fullymap the migration scenario steps.