In his homily, Francis called on people, himself included, to look into their own hearts to see how they are living their lives.
"Has my life fallen asleep?" Francis asked after listening to a Gospel account of how Jesus' disciples fell asleep shortly before he was betrayed by Judas before his crucifixion.
"Am I like Pontius Pilate, who, when he sees the situation is difficult, washes my hands?"
He sounded tired, frequently pausing to catch his breath, as he spoke for about 15 minutes in his homily during Palm Sunday Mass, which solemnly opens Holy Week for the Roman Catholic Church.
"Where is my heart?" the pope asked, pinpointing that as the "question which accompanies us" throughout Holy Week.
Francis seemed to regain his wind after the 2 ½ hour ceremony. He shed his red vestments atop his plain white cassock, chatted amiably with cardinals dressed more formally than he at that point. Then he posed for "selfies" with young people from Rio de Janeiro who had carried a large cross in the square.
He had barely climbed aboard his open-topped popemobile when he spotted Polish youths, they, too, clamoring for a "selfie" with a pope, and he hopped off, to oblige them.
In a crowd of around 100,000 Romans, tourists and pilgrims, people clutched olive tree branches, tall palm fronds or tiny braided palm leaves shaped like crosses that were blessed by Francis at the start of the ceremony.
Francis used a wooden pastoral staff carved by Italian prison inmates, who donated it to him. The pope wants to put people on the margins of life at the center of the church's attention.
Francis wore red vestments, symbolizing blood shed by the crucified Jesus.
Holy Week culminates next Sunday with Easter Mass, also in St. Peter's Square. Many faithful will remain in Rome, while others will pour into the city for the April 27 canonization of two popes, John Paul II and John XXIII. Francis noted that John Paul's long-time aide, now Cardinal Stanislaw Dziwisz of Krakow, Poland, had come to Rome.
Francis noted he'll be making a pilgrimage to South Korea this summer, with the key event, church World Youth Day celebrations on Aug. 15 in Daejeon.