VATICAN CITY — She was known throughout the world as Mother Teresa, considered a saint by many for her charitable work among the poorest of the world’s poor. On Sunday, Pope Francis officially bestowed that title at her canonization ceremony in St. Peter’s Square.

“I think, perhaps, we may have some difficulty in calling her St. Teresa: Her holiness is so near to us, so tender and so fruitful, that we continue to spontaneously call her Mother Teresa,’’ the pope said in off-the-cuff remarks during his homily.

It was a festive atmosphere at the Vatican, under a broiling summer sun, and flags fluttered in the breeze: from Albania, representing the Roman Catholic nun’s ethnic origins; from Macedonia, to note her birthplace, Skopje; from India, where she spent most of her life, working in the slums of Kolkata (formerly Calcutta); and from the many other countries where she touched countless lives.

When Francis proclaimed her St. Teresa of Kolkata at the end of the formal ceremony, in Latin, the crowd erupted in sustained applause.

An estimated 120,000 people gathered in St. Peter’s Square on Sunday, along with 15 official government delegations, including the queen of Spain and representatives of India and the United States. Nuns from Teresa’s order and about 1,500 homeless people also got preferred seating.

The canonization marked a highlight of the Jubilee year, which the pope had proclaimed to celebrate the theme of mercy, and on Sunday he called Teresa a “tireless worker of mercy.’’ The ceremony symbolized the pope’s long emphasis on the church’s role in ministering to those on the margins of society.

Teresa earned fame and accolades over a lifetime spent working with the poor and the sick, and with orphans, lepers, and AIDS patients.

She made the cover of Time magazine in December 1975 for an article that called her one of the world’s “living saints.’’ When told that she had won the Nobel Peace Prize in 1979, she said, “I am unworthy.’’

A portrait of Teresa, once described by Pope John Paul II as an “icon of the good Samaritan,’’ was displayed on the facade of St. Peter’s Basilica and showing her in her distinctive blue-trimmed white sari. The portrait was commissioned by the Knights of Columbus and painted by Chas Fagan, an American artist.

Because of her celebrity, she stepped where many religious figures do not. “She made her voice heard before the powers of this world, so that they might recognize their guilt for the crime of poverty they created,’’ the pope said on Sunday.

Teresa’s supporters praise her selflessness and humility, noting that although she associated with royalty, government leaders, and popes, she continued to live simply until her death in 1997.

“She was one with us,’’ Sister Mary Prema Pierick, superior general of the Missionaries of Charity, said at a Vatican news conference on Friday. “She never wanted or accepted anything not common with all the sisters.’’

The order that Teresa started with 12 nuns now numbers more than 5,800 people in 139 countries, including two orders of brothers and one of priests. The congregation continues her work of ministering to the world’s least privileged, those she called “the poorest of the poor.’’

St. Teresa was canonized 19 years after her death, remarkably fast for modern times.

John Paul II, now also a saint, went against protocol when he allowed the canonization process to begin two years after her death, not the usual five. He beatified her in 2003 after a miracle, the healing of a woman’ with a tumor was attributed to her intercession.

A second miracle, recognized by Francis last year, opened the way to sanctity.

“I am very grateful for this miracle,’’ said Marcilio Haddad Andrino, a Brazilian who recovered from a life-threatening brain infection in 2008 after his family prayed to Teresa. Andrino came to Rome for the ceremony and was present at the Vatican news conference.

“The merciful Lord looks at us all without any distinction,’’ Andrino said. “Maybe it was me this time but maybe tomorrow it will be someone else.’’

Teresa, for all her acclaim, was not without critics. Some have questioned the hygiene and medical standards adopted by the sisters in some of the shelters and clinics run by the Missionaries of Charity.

In a book, and also the documentary “Hell’s Angel,’’ the author and essayist Christopher Hitchens accused Teresa of being an “ally of the status quo,’’ also calling her a “zealot’’ and a “fanatic.’’ Hitchens charged that instead of empowering the poor to seek a better future, she instilled the idea that their condition was permanent.

Her campaigns against birth control and abortion angered feminists and raised concerns with aid organizations.

Some doctors and officials in India have also challenged the narrative of Monica Besra, the woman said to have benefited from Teresa’s first miraculous intervention, saying that Besra had a cyst, not a tumor.

►Many local Catholics stayed up all night to watch live. B1.