The square overflowed with some 250,000 pilgrims, tourists and Romans eager to join the new pope at the start of solemn Holy Week ceremonies, which lead up to Easter.
Keeping with his spontaneous style, the first pope from Latin America broke away several times from the text of his prepared homily to encourage the faithful to lead simple lives.
Palm Sunday recalls Jesus’s entry into Jerusalem but its Gospel also recounts how he was betrayed by one of his apostles and ultimately sentenced to death on a cross.
Recalling the triumphant welcome into Jerusalem, Francis said Jesus “awakened so many hopes in the heart, above all among humble, simple, poor, forgotten people, those who don’t matter in the eyes of the world”.
Francis then told an off-the-cuff story from his childhood in Argentina.
“My grandmother used to say, ‘children, burial shrouds don’t have’ pockets’”.
Since his election on March 13, Francis has put the downtrodden and poor at the centre of his mission as pope, keeping with the priorities of his Jesuit tradition. His name is inspired by St Francis of Assisi, who renounced wealth for austere poverty and simplicity to preach to the poor.
Francis wore bright red robes over a white cassock as he presided over the Mass at an altar sheltered by a white canopy on the steps of St Peter’s Basilica.
In his homily, Francis said Christian joy “isn’t born from possessing a lot of things but from having met” Jesus. That same joy should keep people young, he said.
Francis said he was joyfully looking forward to welcoming young people to Rio de Janiero in July for the Catholic Church’s World Youth Day.
It is the first foreign trip to be pencilled in on the calendar of Francis’s new papacy.