The Vatican spokesman, the Rev. Federico Lombardi, says it was one "one of the most remarkable things" of Francis' first full day on his inaugural international trip.
Lombardi, laughing while recounting the scene, says one nun rushed Francis in a courtyard, embraced him and asked to take a photo with the pontiff.
That blew the lid off and the rest of the nuns ran up to Francis to get their pictures taken, too.
The pope is in Brazil through Sunday for the church's World Youth Day festival.
Copyright 2013 The Associated Press.