Development of mobile software applications for use in specific domains such as Public Security must conform to stringent security requirements. While mobile devices have many known limitations, assuring complex fine-grained security policies poses an additional challenge to quality mobile services and raises usability concerns. We address these challenges by means of a novel approach to authentication and gradual multi-factor authorization for access to sensitive data. Using our mobile Framework that facilitates low-cost composition of rich applications, we have designed and implemented prototype software that provides secure access to information stored in dedicated online systems, such as Customer Relationship Management, collaboration support, and more for Public Sector workers. Usability of our solution has been confirmed by a first evaluation conducted by a group of volunteers.