Hobbled by bad knees and persistent respiratory problems, Francis also didn't participate in the procession of cardinals around the obelisk in the piazza at the start of the Mass. Instead, the 87-year-old pontiff blessed the palm fronds and olive branches carried by the faithful from the altar.

Francis had been expected to deliver a homily halfway through the service and had pronounced the prayers during the Mass. But after several seconds of silence, announcers said Francis had decided not to deliver the homily itself.

Vatican officials estimated some 25,000 people attended the Mass, held under a sunny, breezy spring sky.

Palm Sunday kicks off a busy week for Francis leading up to Easter Sunday when the faithful commemorate the resurrection of Christ. On Thursday, Francis is due to travel to a Rome women's prison for the traditional washing of the feet ritual. On Friday he is scheduled to preside over the nighttime Way of the Cross procession at Rome's Colosseum re-enacting Christ's crucifixion.

The following day marks the Easter Vigil, during which Francis presides over a solemn nighttime service in the basilica, followed by Easter Sunday Mass in St. Peter's Square and his noontime blessing from the loggia above.

Off and on this winter, Francis has been battling what he and the Vatican have described as a case of the flu, bronchitis or a cold. For the last several weeks he has occasionally asked an aide to read aloud his speeches and catechism lessons to spare him the effort.