This paper tests two institutional explanations for why some democratic countries have experienced more anti-government protests than others. The first explanation deals with certain political institutions as structural determinants that shape protest activities, and the second explanation considers opposition parties as agents of protest mobilization. Using a zero-inflated negative binomial regression applied to a global sample of 107 democratic countries from 1990 to 2004, the empirical analyses show that the second explanation works better. The results demonstrate that a larger opposition camp fosters more anti-government protests only if this opposition camp is more united. Moreover, the finding suggests that the mobilization capacity of opposition parties matters for anti-government protests in developing countries but not for those in developed countries.