We have studied the dynamic response of Pb thin films with a square array of antidots by means of ac susceptibility chi(T, H) measurements. For low amplitudes h of the ac excitation, the field dependence of the ac susceptibility exhibits a series of distinctive features associated with the presence of different dynamic regimes. For fields below a certain characteristic field H-s, vortices strongly pinned by the antidots perform an oscillatory movement inside the pinning potential induced by the external ac drive. For fields between H-s and H-p, interstitial vortices, performing an intra-valley motion in the weak potential wells produced by the repulsive interaction of their pinned neighbors, dominate the ac response. For H-p < H < H-t, the interstitial vortices are driven out of their pinning sites while vortices in the antidots remain trapped. For higher fields all vortices are set in motion and a critical state develops. We show that the characteristic h-independent field H-s allows one to clearly identify the maximum number of flux quanta per hole. Unlike H-s, the fields H-p and H-t, signaling the onset of motion of interstitial vortices and vortices trapped by the antidots, respectively, strongly depend on h. (C) 2004 Elsevier B.V. All rights reserved.