Against the backdrop of the popular uprisings on squares around the world, surging on and off from 2008 to 2013, the present contribution offers a critical study of public squares as sites for contested space and urban commoning. Based on an elaborate understanding of differences and similarities between traditional forms of commoning in the open field system and new forms of urban commoning, the multitudinal gathering on Tahrir Square in Cairo at the beginning of 2011 is analysed as a paradigmatic example of the rise of a new public which presents horizontalistic practices of sharing power in common over against authoritarian rulership. As an effect of opposing the prevailing power structures in Cairo, the urban commoners in Tahrir opened up the square to the rest of the city, redefining the use of it and prefiguring new ways of sharing urban space which go beyond the dichotomy of private and public space.