Pope Francis is celebrating Palm Sunday Mass in St. Peter's Square, which is filled with thousands of people waving olive branches and palm fronds.

Pope Francis delivers his blessing to the palms and to the faithful gathered in St. Peter's Square during Palm Sunday Mass (Christopher Furlong/Getty Images)

The new pontiff arrived in an uncovered vehicle to start solemn Holy Week ceremonies, which lead up to Easter, Christianity's most important day.

Francis wore bright red robes over a white cassock and presided over the Mass from an altar sheltered by a canopy on the steps of St. Peter's Basilica.

Cardinals, many of them among the electors who on March 13 chose the Roman Catholic church's first Latin American pope, sat in rows for the ceremony held under hazy skies on a breezy day.