Mother Teresa is to be declared a saint at a ceremony in the Vatican on September 4.
Tens of thousands of people are expected to attend the ceremony led by Pope Francis in St Peter’s Square.
Two miraculous cures of the sick after Mother Teresa’s death in 1997 have been attributed to her intercession.
Mother Teresa was famous for working with the desperately poor in India.
In India, there will be ceremonies at the Missionaries of Charity, the order the Nobel Peace Prize winner founded in Kolkata (Calcutta).
Pope Francis will celebrate a Mass and Canonization in St Peter’s Square at 10:30 local time.
Many pilgrims arrived at the Vatican before dawn on September 4 to get a good spot for the Mass.
Some 1,500 homeless people across Italy are also being brought to Rome in buses to be given seats of honor at the celebration – and then a pizza lunch served by 250 nuns and priests of the Sisters of Charity order.
Mother Teresa founded a sisterhood that runs 19 homes.
However, Mother Teresa was not without her critics, as some people noted a lack of hygiene in the hospitals run by her sisterhood, and said she accepted money from dictators for her charity work.
Mother Teresa died in 1997 at the age of 87 and was beatified in 2003, the first step to sainthood.
In 2002, the Vatican ruled that an Indian woman’s stomach tumor had been miraculously cured after prayers to Mother Teresa.
Pope Francis cleared the way for sainthood in 2015 when he recognized a second miracle attributed to the nun.
Born in 1910 to ethnic Albanian parents as Agnese Gonxha Bojaxhiu, she grew up in what is now the Macedonian capital, Skopje, but was then part of the Ottoman Empire.
At the age of 19, she joined the Irish order of Loreto and in 1929 was sent to India, where she taught at a school in Darjeeling under the name of Therese.
In 1946 she moved to Kolkata to help the destitute and, after a decade, set up a hospice and a home for abandoned children.
Mother Teresa founded the Missionaries of Charity in 1950. The sisterhood now has 4,500 nuns worldwide.
She achieved worldwide acclaim for her work in Kolkata’s slums, but her critics accused her of pushing a hard-line Catholicism, mixing with dictators and accepting funds from them for her charity.
It often takes decades for people to reach sainthood after their death, but beatification was rushed through by Pope John Paul II.
Pope Francis was known to be keen to complete the process during the Church’s Holy Year of Mercy, which runs to November 2016.