Clad in ceremonial gold-embroidered red robes, the pope used the occasion to urge young people to renew their commitment to the Roman Catholic church. Religious faith, he said, can answer "the questions and unrest of every man and every woman, particularly the young."
Palm Sunday, marked by millions of Christians the world over, recalls Jesus' triumphal entry into Jerusalem before his arrest and crucifixion, when the Gospels relate that devout followers strewed palm branches in his path.
In Jerusalem, hundreds of pilgrims staged a Palm Sunday procession that wound its way down the road from Mount Zion to the walled Old City. Some held hands and danced as they made their way along the route.
Sunglasses-wearing pilgrim Dina Dillon of Los Angeles, California, said she had come to Jerusalem to retrace Christ's footsteps.
"This is where people waved their leaves to greet Jesus...(Now) we meet him and greet him with our palms,'' she said.
Ranks of red-robed cardinals carrying palm branches led the procession at St. Peter's Square, where a crowd estimated at half a million spilled from the enormous plaza and into surrounding streets. Visitors--pilgrims and tourists alike--had come from all over the world.
"I wouldn't want to miss seeing this," said Allard Mako, 35, a seminarian from Johannesburg, as he waited to pass through the security gates and metal detectors set up between the soaring columns surrounding the square.
"It's a very, very special time for us," said 20-year-old Francisco Dias of Lisbon, Portugal, dressed in his best and seated with his family.
Tourists snapped pictures of one another, of the Vatican's Swiss Guards in their red-plumed headdresses and pantaloons, and of the pope as he waved to the crowd from his open-air "popemobile."
The pontiff, who turns 80 next month, at times looked drawn and tired during the nearly three-hour Mass. He leaned heavily on his silver staff and his hands trembled, a symptom of Parkinson's disease.
But he appeared energized as he looked ahead to what is expected to be an enormous youth celebration in Rome in August. His voice strong, he told the crowd that Palm Sunday has become a "festival of youth."
The Christian promise of salvation, he told young people, "is not an illusory promise of happiness--on the contrary, with it you can achieve an authentic human and spiritual maturity."
Palm Sunday marked the start of a week of events that will culminate with Easter, when Christians recall Christ's resurrection.
The pope was to preside over a Holy Thursday service, a Good Friday evening procession in Rome's Colosseum, a vigil on Saturday night and an Easter Mass in St. Peter's Square.
The pope has urged Roman Catholic faithful to make pilgrimages to Rome this millennial year, designated by the church as a holy year in keeping with a 700-year-old papal tradition.