The Pope will attend the Catholic youth gathering in Rio de Janeiro, saying Mass on Copacabana beach and praying at the Christ the Redeemer statue.
Pope Francis spoke in St Peter’s Square as Holy Week, the most important period in the Christian calendar, began.
Thousands of people waved olive branches and palm fronds in the crowd.
The gesture commemorates palm branches which, according to the Bible, were laid in the path of Jesus Christ when he made his entry on a donkey into Jerusalem before his crucifixion.
Pope Francis departed frequently from the prepared text of his homily.
His style is informal, direct and, in contrast to his predecessors, he tells stories off the cuff to illustrate the points he wants to make, our correspondent says.
The Catholic youth festival is expected to attract hundreds of thousands of young pilgrims from around the world.
Towards the end of his homily, Pope Francis said: “I look forward joyfully to next July in Rio de Janeiro.
“I will see you in that great city in Brazil.”
The gathering in Rio would, Pope Francis said, be a “sign of faith for the whole world”.
Pope Benedict XVI attended the last such festival, in Madrid, in August 2011.
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