Tourists and pilgrims are already filling the streets around Vatican City for Sunday's canonization of two popes.

Hundreds of thousands are expected to be there for the sainthood ceremony for John Paul II and John XXIII.

Vatican spokesman the Rev. Federico Lombardi says retired pontiff Benedict XVI will be in St. Peter's Square for the canonization of John and John Paul. He says Benedict and many cardinals will "concelebrate" the Mass with Francis.

It will be an unprecedented occurrence of two living popes canonizing two of their predecessors.

Benedict resigned from the papacy a year ago, and since has largely dedicated himself to prayer in a monastery on the Vatican grounds. Sunday's appearance will be his highest-profile one since he retired. Francis, who lives elsewhere in Vatican City, in a guesthouse, has been quite welcoming to his predecessor, occasionally paying a call on Benedict. It was Francis who sought to include Benedict in the canonization ceremony.