VATICAN CITY – Pope Francis celebrated his first Palm Sunday Mass in St. Peter’s Square, encouraging people to be humble and young at heart and promising to go to a youth jamboree in Brazil in July, while the faithful enthusiastically waved olive branches and braided palm fronds.

The square overflowed with a crowd estimated by the Vatican at 250,000 people. Pilgrims, tourists and Romans jostled each other in an eager effort to glimpse Francis as they joined the new pope at the start of solemn Holy Week ceremonies, which lead up to Easter, Christianity’s most important day.

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 and resist the temptation to be sad when life’s obstacles inevitably come their way.

“Don’t let yourselves be robbed of hope! Don’t let yourselves be robbed of hope!” Francis told the crowd, in an apparent reference to the economic difficulties people are grappling with as they try to find adequate work amid a poor job market in much of the world.

At the end of the Mass, Francis took off his red vestments, and wearing his plain white cassock and skull cap, climbed into an open-topped popemobile to circle through the excited crowd. He leaned out to shake hands, kissed and patted the heads of infants passed to him by bodyguards, and often gave children the thumbs-up sign.

His security detail seemed to be reluctantly dealing with this get-close-to-the-people pontiff, scrambling around the vehicle to pick up this child or that one. At one point, the chief bodyguard, Domenico Giani, was sent back to the mother of a child he had greeted to convey a message from the pontiff, and the ever-tense Giani broke into a smile.

Francis even climbed down from the vehicle, kissed a woman in the crowd and chatted briefly with her, and another man in the crowd leaned over a barrier to squeeze the pontiff on a shoulder — an unheard-of familiarity in the previous pontificate of the reserved Benedict XVI.

In keeping with his stress on giving examples of humility, Francis kissed the hand of an elderly woman who had outstretched an arm to him.

“There is no doubt that there will be a new spring for the church, a renewal” with this pope, said Sister Emma, an Argentine nun in the crowd.

Palm Sunday recalls Jesus’ 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.