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, he has suggested he will take a more modest approach than his predecessors.
Pope Francis was elected by a conclave of cardinals last week to take over from Pope emeritus Benedict, reports the BBC.
He 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.
Pope Francis is expected to make a tour of St Peter’s Square to greet the faithful before Mass begins at 09:30 (08:30 GMT).
After delivering a homily, he will be presented with his papal pallium made of lambs’ wool and a gold-plated silver “fisherman’s ring” bearing the image of St Paul holding two keys.
But the Pope has also asked for some of the pomp to be stripped away, the BBC’s Tom Robbins reports from Rome.
The ring is second-hand and made of silver plated in gold, not the solid gold worn by his predecessor, our correspondent adds.
The list of attendees for Monday’s Mass includes US Vice-President Joe Biden and the spiritual head of the Orthodox Church, Patriarch Bartholomew.
He will be the first Orthodox patriarch to attend a papal inauguration Mass since the two branches of Christianity split more than 1,000 years ago.
Thirty-three groups of guests from various Christian Churches are expected in total, according to the Vatican, along with 16 delegations from Jewish communities, as well as representatives of other faiths.
Before his election last week, Pope Francis was Cardinal Jorge Mario Bergoglio of Buenos Aires, and Argentine President Cristina Fernandez de Kirchner will be in attendance at the Mass.
On Monday, Ms Fernandez became the first foreign head of state to be received by Pope Francis and said she had asked for his intervention in the Falklands dispute between her country and the UK.
Up to a million people are expected to attend the Mass in St Peter’s Square
The event will be broadcast on giant screens set up outside the cathedral on Plaza de Mayor, the main square in the capital Buenos Aires.