Pope Francis is marking Palm Sunday Mass in St. Peter's Square, which is jammed with tens of thousands of faithful.

Sprinkling holy water, Francis blessed palm fronds and olive branches clutched by pilgrims, tourists and Romans. He watched from steps at the foot of the obelisk in the center of the square as prelates, holding dried, braided palms, formed a circle around him.

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Pope Francis asperses holy water as he celebrates Palm Sunday Mass in St. Peter's Square at the Vatican, Sunday, April 14, 2019. The Roman Catholic Church enters Holy Week, retracing the story of the crucifixion of Jesus and his resurrection three days later on Easter Sunday. (AP PhotoGregorio Borgia)

Pope Francis is marking Palm Sunday Mass in St. Peter's Square, which is jammed with tens of thousands of faithful.

Pope Francis holds a palm frond as he celebrates Palm Sunday Mass in St. Peter's Square at the Vatican, Sunday, April 14, 2019. The Roman Catholic Church enters Holy Week, retracing the story of the crucifixion of Jesus and his resurrection three days later on Easter Sunday. (AP PhotoGregorio Borgia)

After the blessing, the pope, wearing red robes and cradling a braided palm, joined a procession to an open-air altar to celebrate Mass.

Faithful holding olive branches walk in procession past St. Peter's Basilica during a Palm Sunday Mass celebrated by Pope Francis, in St. Peter's Square at the Vatican, Sunday, April 14, 2019. (AP PhotoGregorio Borgia)

Faithful holding olive branches walk in procession past St. Peter's Basilica during a Palm Sunday Mass celebrated by Pope Francis, in St. Peter's Square at the Vatican, Sunday, April 14, 2019. (AP PhotoGregorio Borgia)

Pope Francis asperses holy water as he celebrates Palm Sunday Mass in St. Peter's Square at the Vatican, Sunday, April 14, 2019. The Roman Catholic Church enters Holy Week, retracing the story of the crucifixion of Jesus and his resurrection three days later on Easter Sunday. (AP PhotoGregorio Borgia)

Pope Francis asperses holy water as he celebrates Palm Sunday Mass in St. Peter's Square at the Vatican, Sunday, April 14, 2019. The Roman Catholic Church enters Holy Week, retracing the story of the crucifixion of Jesus and his resurrection three days later on Easter Sunday. (AP PhotoGregorio Borgia)

Pope Francis arrives to celebrate Palm Sunday Mass in St. Peter's Square at the Vatican, Sunday, April 14, 2019. The Roman Catholic Church enters Holy Week, retracing the story of the crucifixion of Jesus and his resurrection three days later on Easter Sunday. (AP PhotoGregorio Borgia)

Pope Francis arrives to celebrate Palm Sunday Mass in St. Peter's Square at the Vatican, Sunday, April 14, 2019. The Roman Catholic Church enters Holy Week, retracing the story of the crucifixion of Jesus and his resurrection three days later on Easter Sunday. (AP PhotoGregorio Borgia)

Pope Francis, center left, celebrates Palm Sunday Mass in St. Peter's Square at the Vatican, Sunday, April 14, 2019. The Roman Catholic Church enters Holy Week, retracing the story of the crucifixion of Jesus and his resurrection three days later on Easter Sunday. (AP PhotoGregorio Borgia)

Pope Francis, center left, celebrates Palm Sunday Mass in St. Peter's Square at the Vatican, Sunday, April 14, 2019. The Roman Catholic Church enters Holy Week, retracing the story of the crucifixion of Jesus and his resurrection three days later on Easter Sunday. (AP PhotoGregorio Borgia)

Palm Sunday ushers in Holy Week, which includes the Way of the Cross procession at the Colosseum to commemorate Jesus' death by crucifixion. Several solemn ceremonies culminate in joyous celebrations on Easter Sunday as Christians celebrate their belief that Jesus rose from the dead.

Pope Francis asperses holy water as he celebrates Palm Sunday Mass in St. Peter's Square at the Vatican, Sunday, April 14, 2019. The Roman Catholic Church enters Holy Week, retracing the story of the crucifixion of Jesus and his resurrection three days later on Easter Sunday. (AP PhotoGregorio Borgia)

Pope Francis asperses holy water as he celebrates Palm Sunday Mass in St. Peter's Square at the Vatican, Sunday, April 14, 2019. The Roman Catholic Church enters Holy Week, retracing the story of the crucifixion of Jesus and his resurrection three days later on Easter Sunday. (AP PhotoGregorio Borgia)

After the blessing, the pope, wearing red robes and cradling a braided palm, joined a procession to an open-air altar to celebrate Mass.

Pope Francis holds a palm frond as he celebrates Palm Sunday Mass in St. Peter's Square at the Vatican, Sunday, April 14, 2019. The Roman Catholic Church enters Holy Week, retracing the story of the crucifixion of Jesus and his resurrection three days later on Easter Sunday. (AP PhotoGregorio Borgia)

Pope Francis holds a palm frond as he celebrates Palm Sunday Mass in St. Peter's Square at the Vatican, Sunday, April 14, 2019. The Roman Catholic Church enters Holy Week, retracing the story of the crucifixion of Jesus and his resurrection three days later on Easter Sunday. (AP PhotoGregorio Borgia)

Faithful holding olive branches walk in procession past St. Peter's Basilica during a Palm Sunday Mass celebrated by Pope Francis, in St. Peter's Square at the Vatican, Sunday, April 14, 2019. (AP PhotoGregorio Borgia)

Faithful holding olive branches walk in procession past St. Peter's Basilica during a Palm Sunday Mass celebrated by Pope Francis, in St. Peter's Square at the Vatican, Sunday, April 14, 2019. (AP PhotoGregorio Borgia)

Pope Francis asperses holy water as he celebrates Palm Sunday Mass in St. Peter's Square at the Vatican, Sunday, April 14, 2019. The Roman Catholic Church enters Holy Week, retracing the story of the crucifixion of Jesus and his resurrection three days later on Easter Sunday. (AP PhotoGregorio Borgia)

Pope Francis asperses holy water as he celebrates Palm Sunday Mass in St. Peter's Square at the Vatican, Sunday, April 14, 2019. The Roman Catholic Church enters Holy Week, retracing the story of the crucifixion of Jesus and his resurrection three days later on Easter Sunday. (AP PhotoGregorio Borgia)

Pope Francis arrives to celebrate Palm Sunday Mass in St. Peter's Square at the Vatican, Sunday, April 14, 2019. The Roman Catholic Church enters Holy Week, retracing the story of the crucifixion of Jesus and his resurrection three days later on Easter Sunday. (AP PhotoGregorio Borgia)

Pope Francis arrives to celebrate Palm Sunday Mass in St. Peter's Square at the Vatican, Sunday, April 14, 2019. The Roman Catholic Church enters Holy Week, retracing the story of the crucifixion of Jesus and his resurrection three days later on Easter Sunday. (AP PhotoGregorio Borgia)

Pope Francis, center left, celebrates Palm Sunday Mass in St. Peter's Square at the Vatican, Sunday, April 14, 2019. The Roman Catholic Church enters Holy Week, retracing the story of the crucifixion of Jesus and his resurrection three days later on Easter Sunday. (AP PhotoGregorio Borgia)

Pope Francis, center left, celebrates Palm Sunday Mass in St. Peter's Square at the Vatican, Sunday, April 14, 2019. The Roman Catholic Church enters Holy Week, retracing the story of the crucifixion of Jesus and his resurrection three days later on Easter Sunday. (AP PhotoGregorio Borgia)