Pope Francis has remembered those killed in the Germanwings crash as he opened solemn Holy Week services with Palm Sunday Mass in St Peter's Square.

At the end of Mass outside St Peter's Basilica for some 70,000 faithful, Pope Francis prayed for those who died in Tuesday's crash in the French Alps, noting there was a group of German schoolchildren aboard the aircraft.

The disaster killed 150 people, including the co-pilot who investigators say deliberately crashed the plane into the mountain.

Pope Francis clutched a palm frond during the traditional procession at the service's start. In keeping with the simple tone of his two-year-old papacy, he leaned on a plain wooden pastoral staff instead of a traditionally more ornate one as he stood under a red canopy on the basilica steps.

He wore bright red vestments to recall Jesus' death by crucifixion. In his homily, Pope Francis stressed humility, another quality that has marked his papal style.

He hailed those who quietly ignore their own needs to serve others and paid tribute to Christians who endure with dignity humiliation, discrimination and even persecution for their faith.

Many of the faithful, holding olive branches as symbols of peace, are among pilgrims who have poured into Rome for Holy Week appearances by Pope Francis.

On Friday evening, the Pope will preside over a Way of the Cross service at the Colosseum. On Easter Sunday he will celebrate mid-morning Mass in St Peter's Square and give a blessing from the basilica's central balcony.