The nun Mother Teresa, famous for working with the desperately poor in India, is to be declared a saint at a ceremony in the Vatican.
Tens of thousands of people are flocking to St Peter’s Square to see Pope Francis lead the ceremony.
Two miraculous cures of the sick after Mother Teresa’s death in 1997 have been attributed to her intercession.
In India, there will be ceremonies at the Missionaries of Charity, the order she founded in Kolkata (Calcutta).
Pope Francis will celebrate a Mass and Canonisation in St Peter’s Square at 10:30 local time (08:30 GMT).
Many pilgrims arrived at the Vatican before dawn on Sunday 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 honour 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, and won the Nobel Peace Prize.
But she 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.
She died in 1997 – aged 87 – and was beatified in 2003, the first step to sainthood.
In 2002, the Vatican ruled that an Indian woman’s stomach tumour had been miraculously cured after prayers to Mother Teresa.
Pope Francis cleared the way for sainthood last year when he recognised a second miracle attributed to her.
Born in 1910 to ethnic Albanian parents, Agnese Gonxha Bojaxhiu grew up in what is now the Macedonian capital, Skopje, but was then part of the Ottoman Empire.
Aged 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.
She 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 hardline 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.