Crowds are gathering at the Vatican in Rome, with up to a million expected for the inauguration Mass of Pope Francis.
Political and religious leaders from around the world are also attending the Mass in St Peter's Square, marking the official start of his papacy.
The first pontiff from the Americas, Francis has suggested he will take a more modest approach than predecessors.
Francis was elected by a conclave of cardinals last week to take over from Benedict XVI.
Benedict became the first pontiff in 600 years to abdicate last month. Citing his age, 85, he said he could no longer continue in the post.
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