Mother Teresa, the acclaimed nun who served the poorest of the poor in the Indian city of Kolkata all her life, was canonized by Pope Francis in Vatican city.

"For the honor of the Blessed Trinity... we declare and define Blessed Teresa of Calcutta [Kolkata] to be a Saint and we enroll her among the Saints, decreeing that she is to be venerated as such by the whole Church," the Pope said on Sunday morning.

Thousands attended the canonization ceremony of Mother Teresa at St. Peter's square, which declared her as Saint Teresa of Kolkata.

"A saint is an exemplar of holiness for others. The processes of beatification and canonization do not make a saint, but recognizes the spectacular holiness of someone as a model for how to love God above all else and one's neighbor in God through Christ," John O'Callaghan wrote in an opinion on Fox News.

"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 pontiff said in his unscripted remarks.

She fed and comforted thousands of those who were left to die on the streets of Kolkata at Missionaries of Charity which was established in 1950. She gave her life working for the poor till her death in 1997. The organization cares for former prostitutes, the mentally ill, sick and abandoned children, lepers, people with AIDS, and the aged. The charity home includes shelter for women, orphaned children, terminally ill patients, and people with leprosy. The institution also houses schools for street children.

"She bowed down before those who were spent, left to die on the side of the road, seeing in them their God-given dignity," Francis said. "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. For Mother Teresa, mercy was the 'salt' which gave flavour to her work, it was the 'light' which shone in the darkness of the many who no longer had tears to shed for their poverty and suffering."

People had come from different parts of Europe and world, including from her home country Albania and the country of her residence, India, where she had spent most of her life.

She was beatified in 2003 by St. John Paul II.

She began her missionary work with the poor in 1948 and received basic medical training in the Holy Family Hospital, after which she started living in the slums. In 1949, she started a new community to help the poor.

In 1946, while travelling by train, Mother Teresa had an experience which she described as a "call within the call."

"I was to leave the convent and help the poor while living among them. It was an order. To fail would have been to break the faith," she said.

That is when she decided she would leave the comforts of Loreto convent [her former congregation] to live with the poor.

"Our Lord wants me to be a free nun covered with the poverty of the cross. Today I learned a good lesson. The poverty of the poor must be so hard for them. While looking for a home I walked and walked till my arms and legs ached. I thought how much they must ache in body and soul, looking for a home, food and health," she is quoted as saying in one of her recorded memoirs.