While the faithful prayed, vendors sold papal memorabilia and workers installed huge screens in the capital's central Plaza de Mayo in preparation for an all-night vigil ahead of Francis' formal installation ceremony, which will begin before dawn Tuesday Argentina time.
Mass in the slums began earlier and seemed more intimate. At the Villa 21-24, one of the capital city's most dangerous slums, many recalled how as an archbishop, Jorge Mario Bergoglio blessed their tiny Caacupe Virgin of the Miracles chapel.
Not everyone could fit inside its yellow walls with painted saints and many stood outside. Blanca Franco, a 58-year-old housewife, listened and shed some tears.
"I have feelings in my heart that only God knows," Franco said. "I'm happy about the appointment of our archbishop."
Nearby, Marcos Manuel Solar, a 28-year-old former drug addict, smiled widely as he told passers-by that he had received the rite of confirmation from the pope himself. Solar, who works at a soup kitchen, said he kicked the habit through a rehab program funded by the church.
In the heart of Buenos Aires, the mood was festive.
Vendors outside the Cathedral sold pins, calendars and posters with the image of Francis. The flag of the Vatican City seemed to be the most popular sales item.
Uma Eiras, a 10-year-old student, waved a tiny one with its two white-and-gold vertical bands and said she would hang it in her room.
There seemed not to be a single pew empty inside the giant national Cathedral and many followed the Mass on large screens across the street.
Sabina Podrez, a 39-year-old scrivener, said she attended the Mass out of hope that Argentina and the world will change under a new pope.
"We were going to work disheartened, with insecurity," Podrez said. "This is a breath of fresh air."