In this experimental study, we examine the efficacy of a novel anti-corruption approach that synergistically combines top-down institutional measures with grassroots monitoring by the public. Employing a repeated stranger matching harassment bribe game, we introduce a group monitoring mechanism where the likelihood of corrupt officials being punished increases as more citizens engage in monitoring. Our findings highlight that citizens' decisions to monitor are primarily driven by officials' harassment. Despite facing the social dilemma of anti-corruption, a substantial proportion of citizens willingly incur costs to monitor, resulting in a significant reduction in officials' bribe demands. Although an additional crackdown mechanism slightly decreases citizens' intrinsic motivation, it ultimately enhances the efficiency of anti-corruption outcomes over time, demonstrating its long-term effectiveness.