Modern political transformations involve free choice of ideology, ability to communicate with society and maintenance of their political preferences. Political struggle often leads to radical action and political extremism. The aim of this study involved an analysis of political extremism that occurs in modern democracies, and identification of the main factors underlying the development of political extremism. The determinants of the political stability/extremism are analysed based on the algorithm of hierarchical clustering. It is proved that 26 European countries studied in the work can be grouped into four clusters, which are characterised by the number of parties of extremist ideology in the national parliaments of European countries; Elite Quality Index (EQx); the Freedom in the World Index; Political Stability and Absence of Violence Index, which is part of The Worldwide Governance Indicators (WGI). It is revealed that today ideological trends of authoritarian populism, conservatism and extremism are spreading in European countries. Cluster analysis has shown that the political extremism is influenced by the level of quality of national elites, the development of fundamental rights and freedoms, the political stability, and the absence of violence. Further research should focus on econometric simulation of factors shaping political extremism through economic development indicators.