Communication between end-users and spatial databases raises important and specific issues. Unlike classical systems, it requires strong cooperation between two major tools: spatial query language and graphical interface. This paper addresses the problem of building an interactive querying system for spatial databases with an open architecture. We focus on the following issues: (i) design of a database-independent Graphical User Interface (GUI) providing query expression support and advanced user-database interaction functionnalities (ii) software architecture for dealing with exchanges between the database logical representation and a graphical representation at the interface level (iii) implementation of spatial data and functionnalities within a DBMS without requiring changes in the logical data model or query language syntax. We present a prototype coupling the O$_{2}$ DBMS, its query language O$_{2}$Sql, and C++/X-Window-based toolkits as graphical components for the interface implementation.