Addressing faithful in the square, <person>Francis</person> asked for prayers for the gathering of the heads of Catholic bishops' conferences worldwide.
<person>Francis</person> said he wanted the summit, to be "an act of strong pastoral responsibility in the face of an urgent challenge of our time."
Revelations in many countries about priests raping and committing other kinds of sexual abuse against children and a pattern of bishops hiding the crimes have shaken the faith of many Catholics.
They also test the pontiff's ability to ensure the safety of children and punishment for the abusers as well as any complicit superiors.
The Vatican announced Saturday that <person>Francis</person> approved the expulsion from the priesthood for a former American cardinal, <person>Theodore McCarrick</person>, for sexual abuse of minors and adults.
But survivor advocates also have demanded that <person>Francis</person> say what he and other top Vatican officials knew about the prelate's sexual wrongdoing, which spanned decades.
"You abuse a child, you have to be removed from the priesthood," <person>Isely</person> said. "If you cover up for abusing a child, you have to be removed from the priesthood, and this is the only thing that is going to turn the corner on this global crisis."