The Atacama Cosmology Telescope (ACT) has been observing the southern sky in the millimeter range with an angular resolution at the arc-minute level. An analysis of 228 square degrees observed at 148 GHz along a stripe centered at declination -53 degrees reveals the presence of the Silk damping tail in the temperature angular power spectrum of the Cosmic Microwave Background (CMB). This decaying tail becomes truncated by a rising spectrum at scales corresponding to few arcmins (l ~ 3000) whose origin is compatible with a unclustered population of unresolved point sources and some residual anisotropy due to Compton scattering of CMB photons off free electrons (the Sunyaev-Zel'dovich effect). Comparisons with other observations and constraints on different components giving rise to this secondary spectrum are discussed.