Biundini said the high-profile visit will provide a tempting stage for demonstrators looking for a world audience.
"On the one hand, the pope is a very respected figure in Brazil, which is of course a majority Catholic country," he said. "On the other hand, all eyes are going to be on Brazil, so the pope's visit would be a very attractive platform for groups to get their messages across."
For Brazil's government, the pope's visit is seen as a test run for the country's hosting of next year's World Cup soccer tournament and the 2016 Olympics, which will also draw millions of visitors.
In preparation, Brazilian soldiers have invaded a swath of Rio's slums to push out drug gangs and re-establish their authority. In large part because of slum violence, Brazil suffers one of the world's highest homicide rates, which has doubled in the past three decades, according to a new report.
Nonetheless, a recent visit to the slum Francis is set to enter revealed only two police officers parked just a few meters from the chapel where he'll pray. Journalists were warned by adolescent boys to not take pictures of certain areas where drug peddlers were active, standard practice in slums that have yet to be pacified.
Varginha is one of Rio's smallest slums, a triangle-shaped chunk of flat, dusty land sitting between two putrid waterways full of raw sewage. On the third side runs a busy main road with an elevated commuter train that noisily rolls by overhead.
Brazilian police haven't revealed how they'll secure the slum when Francis arrives, beyond that military police will handle the job. The slum's surroundings somewhat ease security concerns, with the waterways acting as natural boundaries and only two roads 300 meters apart from one another allowing access.