Mother Teresa of Calcutta, known as the "saint of the gutters" during her life, was declared a saint of the Roman Catholic Church by Pope Francis on Sunday, fast-tracked to canonization just 19 years after her death.

Tens of thousands of pilgrims packed St. Peter's Square at the Vatican for a service to honor the tiny nun, who worked among the world's neediest in the slums of the Indian city now called Kolkata and become one of the most recognizable faces of the 20th century.

A Nobel Peace Prize laureate, her legacy complements Pope Francis's vision of a humble church that strives to serve the poor, and the festivities in her honor are a highlight of his Holy Year of Mercy, which runs until Nov. 8.