The objective of the study is to contribute to the discourse about media by investigating frames and journalistic techniques used by the corporate mass media to establish boundaries for understanding police and protesters at the 2010 G20 Summit, any temporal changes, and the applicability of the hierarchy of credibility at this international protest event. Using data from 2009 to 2011 in two national and one local newspaper, a frame analysis seeks to uncover how the media frame behaviour and events, what are the primary and second definers of reality, and how the police and protesters are depicted as social problems. The findings suggest that media portrayals of the social actors are framed within an inferential structure that shifts from protester violence before the summits to police violence afterward. Episodic coverage and the decontextualization of people and events create the boundaries for discussion, through the use of the attribution-of-responsibility, conflict, economic-consequences, and human-interest generic frames in the areas of security, social problems, and controversy. We conclude that the results depart from previous research by suggesting a reconceptualization of the hierarchy of credibility. Within conflict and human interest frames, this crisis event destabilized the police as primary definers of crime in favour of citizens and protesters.