We report on the multi-contact frictional dynamics of model elastomer surfaces rubbed against bare glass slides. The surfaces consist of layers patterned with thousands spherical caps distributed both spatially and in height, regularly or randomly. Use of spherical asperities yields circular micro- contacts whose radius is a direct measure of the contact pressure distribution. Optical tracking of individual contacts provides the in-plane deformations of the tangentially loaded interface, yielding the shear force distribution. We then investigate the stick-slip frictional dynamics of a regular hexagonal array. For all stick phases, slip precursors are evidenced and found to propagate quasi- statically, normally to the iso-pressure contours. A simple quasi-static model relying on the existence of interfacial stress gradients is derived and predicts qualitatively the position of slip precursors.