VATICAN CITY (Reuters) - A tired and sombre-looking Pope Francis led a Palm Sunday service before more than 100,000 people, opening two packed weeks of activities including Easter and the canonization of two popes.

The faithful waved palm and olive branches as the 77-year-old Argentine pope, whose pontificate is 13 months old, rode into the square on a white jeep and stopped at the centre of the square to bless palm and olive branches.

The pontiff, wearing red vestments, appeared fatigued at the start of the ceremony and was particularly solemn when he delivered an impromptu homily, putting aside the one he had prepared.

He looked tired and subdued through most of the service but later perked up and seemed energized as he greeted young people holding palm branches, and was driven around the square so more people in the crowd could see him. The Vatican estimated the crowd at more than 100,000.

Palm Sunday marks the day that the Bible says Jesus rode into Jerusalem to the cheers of the crowds the week before Christian believe he rose from the dead.

Francis spoke of the events on the last two days of Jesus' life - his betrayal by Judas, his arrest, beating, trial and crucifixion - and asked his listeners 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.

Pope Francis leads the Palm Sunday mass at Saint Peter's Square in the Vatican April 13, 2014.

For the second straight year, Francis, whose has said the Roman Catholic Church must be closer to the poor and suffering, is holding the event outside one of Rome's basilicas.

Last year he scandalized conservatives by holding it at a youth jail, where he washed and kissed the feet of women and Moslems inmates.

At Sunday's service he walked with a pastoral cross carved from olive wood by the inmates of an another Italian jail.

On Good Friday and Holy Saturday he is due to preside at three services leading up the 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, April 27, he canonizes 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 modernized the Church.

At the end of the Palm Sunday service, young people from Brazil handed over a large wooden cross that was used in the World Day of Youth last July in Rio de Janeiro to those from Poland, where the next one will be held in 2016.