Waving palm fronds and olive branches, tens of thousands of pilgrims and tourists packed St. Peter's Square on a sunny, breezy day.
According to the New Testament, Palm Sunday marks the day Jesus rode into Jerusalem, greeted by cheering crowds bearing palm fronds.
It marks the start of the Holy Week and continues with the commemorations of Jesus' crucifixion on Good Friday and the celebration of his resurrection on Easter Sunday.
Palm Sunday masses are planned throughout Southern California. Three services were scheduled at Cathedral of Our Lady of the Angels, including a Spanish mass at 12:30 p.m.
Monday night begins Passover.
The Associated Press contributed to this report.