Mass in Rome.
Tens of thousands of pilgrims are expected in St Peter's Square for the Mass that marks the start of Holy Week.
Sprigs of olive trees will be distributed to the faithful in remembrance of Jesus' triumphant entry into Jerusalem before his crucifixion.
The run-up to Easter is considered the most important week in the calendar of the Roman Catholic Church.
After Sunday's Mass, the Pope will lead six more liturgies during the week, culminating with the Easter Sunday Mass and Urbi et Orbi blessing.
What the newly-elected Pope says during these services will take on added significance coming at the start of his pontificate, says the BBC's David Willey in Rome.
Observances modified
On Saturday, the Argentine Pope held a first meeting with his predecessor, Pope Emeritus Benedict, who is now living in retirement near Rome.
The BBC's Alan Johnson: "We will never know the details of their talks"
Pope Francis was flown by helicopter to Castel Gandolfo for the private lunch with Pope Emeritus Benedict.
Benedict has lived at the lakeside castle south of Rome since last month, when he became the first pope in six centuries to resign, citing ill health.
Cardinal Jorge Maria Bergoglio, the archbishop of Buenos Aires, was elected to succeed him on 13 March.
Pope Francis has decided to modify some traditional Vatican Holy Week observances.
On Thursday, for example, he will visit a prison for young offenders in a Roman suburb where he will symbolically wash the feet of 12 young prisoners.
In previous years the ceremony was performed by the Pope in Rome's Cathedral of Saint John Lateran with priests symbolising the 12 apostles.
New style
The new Pope chose the name Francis in honour of St Francis of Assisi - the 13th Century Italian saint who spurned a life of luxury to work with the poor.
He has called for the Roman Catholic Church to be closer to ordinary people, especially the poor and disadvantaged.
And, only 10 days into his pontificate, he has made some subtle but significant changes in the lifestyle of the leader of the Roman Catholic Church, says our correspondent.
He dresses very simply, preferring to wear plain black shoes under a simple white habit rather than the red leather loafers and ermine-trimmed cape worn by his predecessor.
The first Latin American Pope spurned a special car to take a bus with his cardinals after he was elected, and insisted on returning to his Rome hotel the next day to pay his own bill.
And Pope Francis places himself on the same level as his guests, rather than greeting them from a throne on an elevated platform, which is seen as a powerful gesture after centuries of Vatican pomp.
The former archbishop of Buenos Aires has also started inviting guests to his early morning Mass - including Vatican gardeners, street sweepers, kitchen staff and maids working at the hotel where he is currently staying.
