A first activity of the pope when he arrived at the basilica was to take part in renaming the square outside the church from St. John Lateran to Blessed John Paul II Square, the Catholic News Agency/Eternal Word Television Network reported.
As he walked down the aisles greeting people, he interacted with disabled persons individually, a personal touch he has shown since becoming pope, the news agency reported.
Francis focused on the theme of God's mercy in his homily on the occasion of Sunday's Feast of Divine Mercy.
The pope noted that the day's Gospel reading of the story of Doubting Thomas is about St. Thomas not believing the news of Jesus rising from the dead without seeing him with his own eyes, the news agency said.
"Jesus tells Thomas to put his hand in the wounds of his hands and his feet, and in his side. We too can enter into the wounds of Jesus, we can actually touch him. This happens every time that we receive the sacraments with faith," the pope said.
"God's patience has to call forth in us the courage to return to him, however many mistakes and sins there may be in our life."