The number of items in a visual image is underestimated when these are symmetrically arranged compared to when they are randomly scattered in space. The neural mechanisms underlying this perceptual illusion remain unclear. In this study, 19 adults viewed arrays of dots varying in numerosity and spatial arrangement while undergoing EEG recording. Searchlight analyses revealed that physical numerosity was decoded from occipito-parietal electrodes as early as 80 ms after stimulus onset, whereas the arrangement of dots (symmetric vs. random) was processed from frontocentral electrodes at ~ 200ms. Representational similarity analyses confirmed early representation of physical numerosity over and above other low-level features (dot size and convex hull). Crucially, perceived numerosity (illusory altered by the symmetrical arrangements) emerged later, at approximately 150 ms from occipitoparietal and central electrodes. This finding suggests that the illusion likely arises from late attention-dependent grouping processes, such as those involved in incremental grouping supporting object segmentation.