Pope Francis led a Palm Sunday mass before more than 100,000 people, opening two packed weeks of activities including Easter and the canonisation of two popes.
The faithful waved palm and olive branches as the 77-year-old pope rode into St Peter’s Square on a white jeep and stopped at its centre to bless palm and olive branches.
The pontiff was particularly solemn when he delivered an impromptu homily, putting aside the one he had prepared.
Francis spoke of the events on the last two days of Jesus' life - his betrayal by Judas, his arrest, beating, trial and crucifixion.
He asked those present to think hard about who they resembled more, those who helped Jesus or those who condemned him, betrayed him or were indifferent to his fate.
"Where is my heart? Who among these people am I like? This question will remain with us all week," he said.
On Holy Thursday, Francis will preside at two services, including one where he will wash and kiss the feet of elderly people in a nursing home to commemorate Jesus' gesture of humility to his apostle on the night before he died.
For the second straight year, Francis, whose has said theRoman Catholic Church must be closer to the poor and suffering, is holding the event outside one of Rome's basilicas.
Last year he held the service at a youth jail, where he washed and kissed the feet of women and Muslim inmates.
At today’s mass he walked with a pastoral cross carved from olive wood by the inmates of another Italian jail.
On Good Friday and Holy Saturday he is due to preside at three services leading up Easter Sunday, when he delivers his "Urbi et Orbi" (to the city and the world) blessing and message.
The pope would normally rest for most of the week after Easter but this year Rome will be full of visitors flocking to the Italian capital from around the world to see him declare two of his predecessors saints.
On Sunday, 27 April, he will canonise Pope John Paul II, who reigned from 1978 to 2005, and Pope John XXIII, who was pontiff from 1958 to 1963 and called the Second Vatican Council, a landmark meeting that modernised the Church.