This thesis summarizes characteristics of common models of multilevel database security which are based on mandatory access data security. Multilevel security models (MLS models) have been evolving in time. Soon after the first MLS model is published, it has been clear that it contains some semantic flaws in its structure. Since then, every new MLS model tried to remove semantic problems that his ancestor had. Since vast majority of these problems came due to the lack of semantic rules, I paid special attention to the semantics in MLS models in my thesis. After comparison of existing MLS models, the conclusion was that the most complete MLS model was Belief-Consistent Multilevel Secure Relational Model (BCMLS model) so I decided to build my prototype based on that model. Additional motive for me was the fact that BCMLS model was never implemented as prototype nor as a commercial product. During implementation I focused on implementing basic operations for selecting and manipulating of data in database and implementation special functions of BCMLS model (such as, for instance, explicit verify operation). In the end I noted the problems I got into while implementing BCMLS prototype, lack of commercial solutions, limits of DBMS in relational database systems for implementing MLS models, security and functional aspects, issues with development and maintenance of BCMLS system.