According to the group engagement model of social justice, being treated fairly in a group, organization, or society enhances willingness to advocate for collective interests. At the same, according to the system justification theory, system justification motivation can increase perceptions of procedural justice and consequently lower the motivation for behaviours aimed at social change. In this study, we will explore whether political procedural justice and system justification motivation (belief in a just world) together with internal political efficacy (interactively) explain previous protest political participation and voting after controlling for participants’ gender, age, residence type and education. The analyses of data from the ESS Round 9 (2018/2019) were conducted separately on two groups of European countries (17 European old democracies and 12 European new democracies (with the exclusion of Cyprus) by using structural equation modelling (SEM) with cluster-robust standard errors (using countries as clusters) and the double mean-centred product-indicator approach to operationalizing latent interactions in R. In both types of countries individuals with higher political self-efficacy self-reported higher activism and voting. Participants perceiving stronger procedural injustice were slightly more likely to engage in activism (but not voting) in old democracies, while no such relationship was established in the new democracies. Participants exhibiting a higher belief in a just world were less likely to participate in activism and voting in old democracies. In new democracies, however, such participants were only slightly less likely to engage in activism. Additionally, the analyses yielded a significant three-way interaction in old democracies in the case of activism: among participants with a low and moderate belief in a just world, individuals perceiving more procedural injustice reported higher levels of activism if their political self-efficacy was high. Contrarily, among participants high on belief in a just world, individuals below average or average on political self-efficacy exhibited higher activism as they perceived more procedural injustice. No relationship between procedural injustice and activism was established among individuals believing in a just world and who perceived themselves as high on political efficacy. Only the interaction between belief in a just world and political self-efficacy in the prediction of activism was confirmed in the sample of new democracies – positive relationship between political self-efficacy and activism was stronger among individuals exhibiting lower belief in a just world. On the other hand, a significant interaction between perceived procedural injustice and belief in a just world was established in the case of voting. Among participants with lower beliefs in a just world, higher perceived injustice is related to lower electoral past participation. In contrast, among those believing in a just world, higher perceived injustice is related to higher electoral past participation. These cross-country quantitative results regarding the role of perceived justice in new democracies are expanded with qualitative findings based on semi structured interviews conducted in 2024 in Croatia with activist and members of general population.