Research on the safety of large mass sporting events of an international dimension is not common in Poland, because the results of preliminary research clearly indicate that they are most often of an ad hoc nature and focus on smaller mass sporting events organized in the country. In this regard, the most frequently discussed issues in the studies are related to the safety of football matches as sports events with an increased level of risk. In addition, most studies are fragmentary rather than comprehensive. This turned out to be important due to the fact that in practice the organization of such large sports events turns out to be extremely difficult and demanding, as evidenced by the study presented in the article on the organization of the Olympic Games in London (2012) and Rio de Janeiro (2016). Research has confirmed that most of the risks are common to all countries organizing major sporting events, but some of them result from the internal situation, geographical location or other factors. It should be emphasized that the research focused on the key factors that determined the stages of preparation, organization and operation of the security system during the Olympic Games in London and Rio de Janeiro. As a result of the conducted research, it can be assumed that the armed forces play a very important role in the security system of large mass sports events, which, due to their capabilities and specialized equipment, are a very important element of it. In this regard, the armed forces are particularly useful in protecting the airspace and sea basins against terrorist attacks. Bearing in mind the presented facts and the adopted research assumptions, it was assumed that the purpose of the article will be: to present the main security measures that had to be carried out by the organizers of large mass sports events such as the Olympic Games in London and Rio de Janeiro. The need to achieve the assumed goal was a contribution to organizing further steps and formulating the main research problem, which took the form of a question: What security measures have been taken by the organizers of large mass events such as the Olympic Games in London and Rio De Janeiro? It was possible to solve the main research problem after verifying the working hypothesis: it was assumed that such large mass sports events as the Olympic Games would be threatened not only by terrorist attacks, but also by other criminal acts. Under the conditions of the forecast threat, countries organizing such sports events are obliged to take a number of financial, legal, organizational and technical measures that will ensure the necessary level of safety for all participants. Very often, actions taken for security purposes take the form of extensive international cooperation. For the purpose of fulfilling the adopted research assumptions, appropriate theoretical methods of scientific research were used, i.e. analysis, synthesis, comparison and analogy, and inference.