Pope Francis today proclaimed Mother Teresa a saint, hailing her work with the destitute of Kolkata as a beacon for mankind and testimony of God's compassion for the poor.

The revered nun's elevation to Roman Catholicism's celestial pantheon came in a canonisation mass in St Peter's square presided over by Pope Francis in the presence of 100,000 pilgrims.

Mother Teresa's elevation to Roman Catholicism's celestial pantheon came in a canonisation mass in St Peter's square in the Vatican that was presided over by Pope Francis. Photo: AFP

"For the honour 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 pontiff said in Latin.

Francis said that even though the nun had been declared a saint, she would always be Mother Teresa to the Catholic family.

Echoing his own vision of a "poor church for the poor", the pope described Teresa's work as "eloquent witness to God's closeness to the poorest of the poor."

To applause, he added: "Mother Teresa loved to say, 'perhaps I don't speak their language but I can smile'.

"Let us carry her smile in our hearts and give it to those whom we meet along our journey, especially those who suffer."

Francis also used his sermon to recall Teresa's fervent opposition to abortion, which she termed "murder by the mother" in a controversial Nobel Peace prize speech in 1979.

Pizza for the poor

The ceremony came a day before the 19th anniversary of Teresa's death in Kolkata, the Indian city where Teresa spent nearly four decades working in wretched slums.

With the 16th century basilica of St Peter's and an azure sky providing the backdrop, the faithful basked in the late summer sun as Francis presided over a ritual mass that has barely changed for centuries.

Such was the demand from pilgrims, the Vatican could easily have issued double the number of tickets but for space and security restrictions.

Polish faithful attend the holy mass and canonisation of Mother Teresa, on St. Peter's Square in the Vatican, on September 4, 2016. Photo: AFP

Helicopters had buzzed overhead earlier, testifying to a huge security operation. Some 3,000 officers were on duty to ensure the day passed off peacefully for the pilgrims and scores of dignitaries from around the world.

Among the crowd were some 1,500 people who are helped by the Italian branches of Teresa's order, the Missionaries of Charity.

After the mass they were to be Francis's guests at the Vatican for a giant pizza lunch served by 250 sisters and 50 male members of the order.

Teresa spent all her adult life in India, first teaching, then tending to the dying poor.

It was in the latter role, at the head of her now worldwide order that Teresa became one of the most famous women on the planet.

Born to Kosovan Albanian parents in Skopje -- then part of the Ottoman empire, now the capital of Macedonia -- she won the 1979 Nobel Peace Prize and was revered around the world as a beacon for the Christian values of self-sacrifice and charity.

But she was also regarded with scorn by secular critics who accused her of being more concerned with evangelism than with improving the lot of the poor.

Disputed legacy

The debate over Teresa's legacy has continued after her death, with researchers uncovering financial irregularities in the running of her order and evidence mounting of patient neglect, insalubrious conditions and questionable conversions of the vulnerable in her missions.

By historical standards, Teresa has been fast-tracked to sainthood. John Paul II was a personal friend and as the pope at the time of her death, he was responsible for her being beatified in 2003.

Achieving sainthood requires the Vatican to approve accounts of two miracles occurring as a result of prayers for Teresa's intercession.

The first one, ratified in 2002, was of an Indian woman, Monica Besra, who says she recovered from ovarian cancer a year after Teresa's death -- something local health officials have put down to medical advances rather than the power of prayer.

In the second, approved last year, Brazilian Marcilio Haddad Andrino says his wife's prayers to Teresa led to brain tumours disappearing. Eight years later, Andrino and his wife Fernanda were in the congregation on Sunday.

Also in the crowd at St Peter's was Teresa Burley, an Italy-based American teacher of children with learning difficulties, who said the new saint inspired her vocation.

"I remember growing up admiring the things she did for children and the poor," she told AFP.

"We need to remember we are here to help each other. We need to be here for those who can't help themselves."

Many Indians have made the trip to Rome, among them Abraham, an expatriate living in London, who said Teresa's life had set a unique example to the world.

"She practised Christianity. The majority of Christians only spend their time talking about it," he said.