The purpose of this paper was to systematize the system of rescue undertaking which is realized during the mass events by a particular emergency service. To explain the issue, authors designed the object model of emergency activities during a mass event in the form of the system which is especially recruited to minimize the negative effects of such events. Besides the basic participants of rescue undertaking system was identified. Cybernetic model of rescue undertaking system during mass events was designed. The conducted analyses allowed to identify the decision-making processes which flow between the system organization levels which are called: executive, tactical and operative subsystem are shown. It constitutes the basis to specify the requirements of computer support for emergency system during mass events. The basic rules of proceeding by emergence services at mass event’s areas were described. The rescue activities will be a little different when sources of danger are still active and not. Then in the first case the most important quest of rescue undertaking system within rescue activities is separate the danger zone at mass event’s area. In the second case it is not necessary. As a result of the analysis authors designed an activity diagram (in UML language), which show particular rescue activities during the mass event with division responsibilities on each emergency functional employee.