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Alessandra Tarantino, Associated Press
A Missionaries of Charity nun arrives in St. Peter's Square at the Vatican to attend a jubilee audience for workers and volunteers of mercy, Saturday, Sept. 3, 2016. The square hosts a ceremony Sunday expected to draw hundreds of thousands of admirers of Mother Teresa, a nun who before her death in 1997, cared for the destitute who were dying in the streets of India.

VATICAN CITY — Pope Francis on Saturday denounced what he called the modern-day sin of indifference to hunger, exploitation and other suffering, while commending the example of Mother Teresa on the eve of a sainthood ceremony for the nun who cared for India's destitute.

"Tomorrow, we'll have the joy of seeing Mother Teresa proclaimed a saint," Francis told thousands of lay volunteers in St. Peter's Square at a special gathering to stress the need for more mercy and caring in the world. "She deserves it!"

Francis will lead a Sunday morning canonization ceremony in the square which is expected to draw huge crowds of faithful and other admirers of Mother Teresa, who founded an order of nuns devoted like her to giving tenderness and assistance to the poor who were sick and dying in the streets of Kolkata.

Cheering the pontiff in Saturday's crowd were many nuns from her Missionaries of Charity order, each wearing the characteristic white sari trimmed in blue that makes them easily identifiable worldwide where they care for the needy. Francis greeted a group of these nuns as he was driven through the square in his popemobile, and one of the nuns put a blue-and-white garland around his neck.

In his speech to the volunteers, including some who helped rescue survivors of the Aug. 24 earthquake in central Italy, Francis decried those who "turn the other way not to see the many forms of poverty that begs out for mercy."

Choosing "to not see hunger, disease, exploited persons, this is a grave sin. It's also a modern sin, a sin of today," he said.

Francis hailed volunteers as "artisans of mercy," whose hands, voices, closeness and caresses help people who suffer feel loved. While in the square, he petted Leo, the Labrador which pinpointed a 4-year-old child who had survived in a pile of quake rubble. The dog raised a paw, which Francis grasped.

Since becoming pope in 2013, Francis has been encouraging Catholic faithful and institutions to tend to the needs of marginalized people.

"The credibility of the church lies, in convincing part," on the service of Catholics to the poor, the homeless, prisoners, immigrants, refugees and others in need, Francis said. "The world needs concrete signs of solidarity, above all when faced with the temptation toward indifference."

In a shop in Kolkata which sells snacks and rosaries, Muslim shopkeeper Tanveer Ahmed recalled seeing Mother Teresa and other nuns take in a leprosy patient who lay bleeding in the street while others passed by, unmoved.

"We are fighting with each other. We are killing each other. But, if you want to see love, please look at Mother Teresa," Ahmed said. He added: "I believe Mother is next to God."

Rishi Lekhi and Rishabh Jain contributed from Kolkata, India.

Frances D'Emilio is on twitter at www.twitter.com/fdemilio