Pope starts Easter with Palm Sunday mass

Pope Francis has kicked off week-long observances leading up to Easter by celebrating Palm Sunday mass with some 100,000 pilgrims in a sun-drenched St Peter's Square.

Dressed in Palm Sunday red, the head of the Roman Catholic Church led a traditional procession of palms along with scores of cardinals, bishops and priests.

For the observance recalling Jesus' triumphal entry into Jerusalem shortly before his crucifixion and resurrection, worshippers carry palm and olive branches, which the Gospels says were used to greet Jesus when he entered Jerusalem.

In his homily, in which the Argentinian pontiff frequently strayed from a prepared text, he asked: "Where is my heart? ... Am I like Judas? Capable of betraying Jesus, (or) like the disciples who, understanding nothing, slept while he was suffering?"

Francis urged the faithful to ask themselves if they were like "the other disciples who wanted to resolve everything by the sword", or "like Judas, who pretended to love Jesus, kissing him before betraying him."

After the mass, the 77-year-old pontiff toured the throng aboard his white open-top popemobile, posing for "selfies", kissing babies and enjoying a sip of mate - a herbal tea traditional in his native Argentina - offered to him by a pilgrim.