The first American pope was formally taking possession of two papal basilicas in the Eternal City, a formality that serves to ceremonially introduce Leo to his Roman flock.
One of the many titles that Leo assumed when he was elected May 8 was bishop of Rome.
Given his responsibilities running the 1.4-billion strong universal Catholic Church, popes delegate the day-to-day governance of running of the diocese of Rome to a vicar.
Sunday's ceremonies at the St John Lateran and St Mary Major basilicas follow Leo's visit last week to the St Paul Outside the Walls basilica.
Together with St Peter's Basilica in the Vatican, the four papal basilicas are the most important in the West.
Rome's Mayor Roberto Gualtieri welcomed Leo first at the steps to city hall, noting that his May 8 election fell during a holy year, an event occurring every 25 years to invite pilgrims to Rome.
The city underwent two years of traffic-clogging public works projects to prepare and expects to welcome upwards of 30 million people in 2025.
Leo said he felt the "serious but passionate responsibility" to serve all Romans during the holy year and beyond.
Wearing his formal papal garb, Leo recalled the words he had uttered from the loggia of St Peter's Basilica on the night of his election.
The Augustinian pope quoted St Augustine in saying: "With you I am Christian, and for you, bishop."
"By special title, today I can say that for you and with you I am Roman!" he said.
Leo, 69, the former Robert Prevost, replaced Pope Francis, the first Latin American pope, who died April 21 and is buried at St Mary Major, near a beloved icon of the Madonna.
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