Recent advances in communication networks and cryptographic techniques have made possible to consider online voting systems as a feasible alternative to conventional elections. Until today several protocols for electronic voting have been proposed, unfortunately only a few of them have been implemented in an end-to-end fully functional system. In this paper we present a secure electronic voting system for medium scale on-line elections (SELES). Our system efficiently implements a security communication protocol offering protection against double voting and others frauds while avoiding any private voting channel. SELES accomplishes all the standard properties of conventional voting systems, namely, accuracy, democracy, privacy, verifiability, simplicity, flexibility and double voting detection. Our system has been tested in a distributed and heterogeneous Internet network comprised by workstations, laptops and PDA nodes interacting through wired and wireless connections. Additionally, SELES has been designed to deal with communication failures, thus achieving a certain degree of robustness.