In a service to mark Christ's entry into Jerusalem a week before being crucified, Benedict urged the faithful to shun greed and selfish egoism.
"Is our faith pure and open enough?" asked the Pope, seated on a dais in front of St. Peter's basilica, wearing red, gold and white vestments. "Do we perhaps let idols, through various means, enter in the world of our faith?"
Speaking to thousands of pilgrims packed in St. Peter's Square on a cloudy day, the Pope recalled Christ's anger on finding money changers and merchants in a Jerusalem temple courtyard as a reminder of greed that must be rooted out.
Presiding over the third Palm Sunday of his pontificate -- John Paul II died just after Easter in 2005 -- Benedict led a procession of cardinals and bishops carrying palm fronds in St. Peter's Square to start the celebrations.
He then blessed palms and olive branches -- symbols of peace -- with holy water. He closed celebrations with an emotional appeal for an end to violence in Iraq, recalling the death of a kidnapped Chaldean Catholic archbishop whose body was found last week in the northern city of Mosul.
Palm Sunday marks the start of Holy Week, with the eight days leading to Easter Sunday the most intense in the Roman Catholic Church's liturgical calendar.
On Holy Thursday, the Pope will preside over two masses recalling Christ's Last Supper with his apostles.
On Good Friday he will hold two services commemorating the passion and death of Christ, including a Via Crucis (Way of the Cross) procession around the ancient ruins of Rome's Colosseum.
Benedict then presides at an Easter Eve mass on Saturday and caps the week off with Easter Sunday celebrations in St Peter's Square when he delivers his twice yearly Urbi et Orbi (to the city and world) blessing and message.
