Pope Francis has ended his inaugural homily with a request to his Christian followers to "pray for me".
He said his role as the Bishop of Rome is to bring hope by service to their faith.
The "authentic power is service" he said, adding that kindness and compassion is "not a sign of weakness".
Pope Francis has received applause as he paid tribute to his predecessor Benedict XVI during his homily, given in Italian, in front of the audience at St Peter's Square.
He sent Benedict XVI "prayers, affection and thanks".
Marking the feast of St Joseph today, Pope Francis said Christians must listen to God to "protect Christ in our lives so we can protect others".
Today's ceremony at St Peter's Square in Rome is being streamed live:
A young boy with a beautiful voice has sung a solo to the gathered crowds in St Peter's Square, and the millions watching worldwide. No pressure...
Pope Francis collected the key Papal symbols for his inauguration from the tomb of St Peter, under the high altar of the Basilica.
Cardinals then streamed out into St Peter's Square, followed by Pope Francis, for the start of the public ceremony.
Pope Francis appeared to thoroughly enjoy his drive through the crowds in St Peter's Square on his way to the Basilica.
Crowds at the Vatican are certainly smaller than authorities had planned for the inauguration.
Up to a million had been forecast to flock to Rome for the ceremony.