The electrodeposition of Ag on HOPG (0001) substrate is studied using conventional electrochemical methods (cyclic voltammetry, chronoamperometry) and in situ scanning tunnelling microscopy (STM). Due to a weak substrate-deposit interaction the deposition of kg occurs according to a Volmer-Weber island growth mechanism. At relatively low overpotentials the electrodeposition reaction is mainly initiated on steps and other surface defects. The initial deposition kinetics can be described by a model involving progressive nucleation on active sites and diffusion-controlled 3-D growth. The analysis of experimental results shows that the 4-atomic Ag cluster represents the critical nucleus in the overpotential interval -35mV < eta < -10mV. It is demonstrated that local nanoscale deposition of Ag on flat HOPG (0001) terraces can be induced using STM technique and specific pulse polarisation routines.