This study aims to clarify the role of actionism as a peacebuilding tool, identify the advantages and disadvantages of its various techniques (performance, happening, art installation, flash mob, etc.), and evaluate their effectiveness in the urbanized space. The research methodology is based on postmodernism and its comprehension of activism, public action and protest. Anti-war actionism is considered an element of the system of socio-political actionism. It is characterized as a set of spectacular forms of non-violent public protest against armed aggression and its consequences. The expansion of today’s anti-war actionism beyond the narrow artistic environment and its entrance into the broad social dimension is demonstrated. The empirical basis for the conceptualization of anti-war actionism includes two groups of actions: (1) anti-war actions carried out since the 1960s in the United States and Western Europe; (2) anti-war actions carried out in various countries in response to the Russian invasion of Ukraine in 2022. The regional features of anti-war actionism under the conditions of armed conflicts and wars are determined, and the advantages and disadvantages of anti-war actionism as a peacebuilding technology are identified within the space of modern cities.