Holy See Press Office director Father Federico Lombardi said that as many as 150 cardinals and 1,000 bishops will concelebrate the April 27 canonization Mass for Pope John Paul II and Pope John XXIII.

The concelebrants at the altar will include Vicar General of Rome Cardinal Agostino Vallini, former John Paul II aide Cardinal Stanislaw Dziwisz of Krakow, and Bishop Francesco Beschi of Bergamo, Vatican Radio reports.

Fr. Lombardi told an April 24 press conference that Mass attendees will include more than 93 official delegations from countries and international organizations, plus 24 heads of state. Representatives of other major religions, including Orthodox Christian, Anglican, Jewish and Muslim officials, will attend.

Celebrations of the canonizations include a series of April 26 prayer vigils held in at least 11 churches around Rome.

The Canonization Mass itself will be held in St. Peter's Square on the morning of April 27 at 10 a.m. Rome time. A recitation of the Chaplet of Divine Mercy will begin at 9 a.m., while choirs from Karkow, Rome and the Sistine Chapel will perform at 9:30 a.m.

The canonization is expected to draw millions to Rome.

The Holy See Press Office director said that the prayer vigils will be attended by official delegations from more than 90 countries, with 24 heads of state or royalty.

Fr. Lombardi also noted that Pope Emeritus Benedict XVI was of course invited to attend the Canonization Mass and would be "very welcome." However, he added, "we will have to wait and see."

Benedict XVI resigned the papacy in February 2013 due to his advanced age and deteriorating strength.