Pope Francis has called on the faithful to pray for a summit this coming week at the Vatican to look at the issue of sex abuse.

The Pontiff called the problem an "urgent challenge of our time". He's summoned bishops to Rome to help him chart a way forward after decades of abuse by priests and prelates and systematic cover-ups by their superiors.

The Pope addressed worshippers in St. Peter's square:

"From Thursday to next Sunday there will be a meeting in the Vatican of the presidents of all Episcopal Conferences on the theme of the protection of minors in the church. I invite you to pray for this meeting, which I wanted as an act of strong pastoral responsibility to face an urgent challenge of our time".

But Marco Politi, author of 'Francis Among the Wolves' and a longtime Vatican watcher, said the pope is facing "serious opposition" within the church:

"There is a struggle going on between the pope and his supporters who want a change, and a lot of people amongst the bishops and amongst the clergy who don't want transparency and applying law and order in the abuse issue in the world".

Sunday's events came just a day after a former U.S. Cardinal Theodore McCarrick was expelled from the priesthood after being found guilty of sexual abuse against minors and adults.