Half a million people are expected to attend the canonisation of Mother Teresa at the Vatican on Sunday, in a ceremony transmitted live to her adopted home of Kolkata and Catholic audiences worldwide.

The two-hour mass in St Peter’s Square, led by Pope Francis almost 19 years after she died, will transform the diminutive nun who became a global icon for her work with the poor into Saint Teresa of Kolkata. But it will also reignite deep criticism of the order she founded, the Missionaries of Charity, which according to detractors focused on the elevation, rather than the relief, of suffering.

Pilgrims will venerate her relics and have the opportunity to buy 1.5m commemorative 95c postage stamps, released on Friday, that celebrate her “great strength, simplicity and extraordinary humility … [and] tireless dedication”, according to an accompanying brochure.

In the lead-up to Sunday’s mass, images of Mother Teresa have been publicly displayed in and around the Vatican. A series of seminars, feasts, musical events and prayer sessions held for visiting pilgrims have emphasised the parallels between her life’s work and Pope Francis’s central message of social justice and humility. British pop star Rita Ora is due to perform at the Vatican ahead of the canonisation ceremony.



In Kolkata, three months of commemorations are planned, including book launches, art shows, a stadium mass and the installation last week of a lifesize bronze statue of the nun.



Facebook Twitter Pinterest Pope John Paul II holds hands with Mother Teresa in Kolkata, 3 February 1986. Photograph: Luciano Mellace/Reuters

The prime minister of India, Narendra Modi, paid tribute to Mother Teresa in a radio broadcast, saying “she devoted her whole life to the poor”. He added: “When such a person is conferred with sainthood, it is natural for Indians to feel proud.”

In a letter to the Vatican, Congress president Sonia Gandhi said every Indian, not just the country’s 20 million Catholics, took “immense pride and joy” in the canonisation of a “woman who was the very embodiment of boundless compassion, mercy and grace”.

Critics however have protested against Modi’s decision to send a 100-strong delegation, led by foreign minister Sushma Swaraj, to Sunday’s mass. An online petition said: “It boggles the mind that the foreign minister of a country whose constitution exhorts its citizens to have scientific temper would approve of a canonisation based on ‘miracles’.”



Hindu nationalists have claimed that Mother Teresa was a “soul harvester” who proselytised among the poor, and that she and her followers surreptitiously baptised the dying without their knowledge.



Aroup Chatterjee, a doctor, grew up in Kolkata and now works in the UK. He is one of Mother Teresa’s most vocal critics. “Many rogues have become Catholic saints,” he said. “What bothers me is that the world makes such a song and dance about a superstitious, black magic ceremony.”



He added: “It’s obvious that people are duped, they have a herd mentality. But the media has a responsibility not to collude with it.”



He has described Mother Teresa as “a medieval creature of darkness” and a “bogus and fantastic figure” who went unchallenged by the world’s media.



According to his 2003 book, Mother Teresa: The Final Verdict, based on the testimonies of scores of people who worked with the Missionaries of Charity, the medical care given to sick and dying people was negligible. Syringes were reused without sterilisation, pain relief was non-existent or inadequate, and conditions were unhygienic. Meanwhile, Mother Teresa spent much of her time travelling around the world in a private plane to meet political leaders.



Similar criticisms were made by the late writer Christopher Hitchens in his book, The Missionary Position. Mother Teresa was, he wrote, a “religious fundamentalist, a political operative, a primitive sermoniser, and an accomplice of worldly secular powers”.



The focus of the nun’s work, he said, was “not the honest relief of suffering but the promulgation of a cult based on death and suffering and subjection”.



Among those cited by Hitchens was Susan Shields, a former worker with the Missionaries of Charity, who claimed that vast sums of money accrued in bank accounts but very little was spent on medical expertise or making the lives of the sick and dying more comfortable.



Robin Fox, the editor of the Lancet, wrote in 1994 about the “haphazard” approach to care by nuns and volunteers, and the lack of medically trained personnel in the order’s homes.



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The investigative journalist Donal Macintyre spent a week working undercover in a Missionaries of Charity home for disabled children in Kolkata in 2005. In an article in the New Statesman, he described pitiful scenes. “For the most part, the care the children received was inept, unprofessional and, in some cases, rough and dangerous.”



Three years ago, a study by academics at the University of Montreal concluded that the Vatican had ignored Mother Teresa’s “rather dubious way of caring for the sick, her questionable political contacts, her suspicious management of the enormous sums of money she received, and her overly dogmatic views regarding … abortion, contraception and divorce.”



Mother Teresa was born Anjezë Gonxhe Bojaxhiu in 1910 in what is now Macedonia. She became a nun at the age of 18, and in 1946 received what she described as a “call within a call” to work and live among the poor. Two years later she moved to Kolkata where she was based for the rest of her life.



In 1950, she established the Missionaries of Charity with 12 followers. Today, the order has 5,600 members, and hundreds of thousands of lay volunteers, and runs orphanages, schools, homes for the sick and dying, shelters for the homeless, health clinics and other services in 139 countries.



In 1979, Mother Teresa – by then a globally recognised figure – was awarded the Nobel peace prize. She said she did not deserve the award but accepted it “in the name of the hungry, the naked, the homeless, of the crippled, of the blind, of the lepers, of all those people who feel unwanted, unloved, uncared for throughout society.”



Facebook Twitter Pinterest Mother Teresa carries a child in Beirut in August 1982. Photograph: Dominique Faget/AFP/Getty Images

In her acceptance speech, she focused on abortion, a trademark theme. “Peace is threatened by abortion,” she said. “Today, abortion is the worst evil and the greatest enemy of peace … because if a mother can kill her own child, what will prevent us from killing ourselves, or one another? Nothing.”



After several years of ill health, Mother Teresa died on 5 September 1997 aged 87, and was given a state funeral by the Indian government. Demands for her canonisation began almost immediately.



Two years after her death, the Vatican began the process of beatification, the first stage of becoming a saint. In 2002, the Vatican recognised the “miracle cure” of an Indian woman who had prayed to Mother Teresa about her cancer, though the woman’s husband and doctors said the cancer had been treated with drugs.



Last year, Pope Francis recognised a second miracle attributed to Mother Teresa, clearing the way for her canonisation.



Michael Safi in Delhi contributed to this report

Five steps to sainthood

Mother Teresa will be the 640th saint canonised since 1963, reflecting a huge increase in the number of saints created by modern popes. In the previous 375 years, only 218 saints were canonised.