When is advice obligatory upon witnessing a reprehensible act in public places: Is it generally obligatory, or only when the reprehensible act occurs near me?

It is not obligatory to approach everyone who commits an evil act to denounce it. However, if an evil act is witnessed and it is easy to denounce it without hardship or a greater harm, then one should denounce it. A person cannot denounce every evil act he sees in public places due to the abundance of evil acts there. Therefore, he should fear Allah as much as he can and denounce what he is able to without hardship, seeking the best approach.