The Albanian-born nun, who has ministered to Calcutta's poor for 46 years, is battling against malaria, which causes her fragile heart to stop every time the fever rises. If she lives, she will be 86 on Tuesday.
Despite the prayers, Mother Teresa was "sinking fast", one doctor said. "We put her off the respirator but she worsened immediately," he added.
Twice before in recent years she has fallen critically ill and rallied. She was fitted with a pacemaker in 1992. But this time, doctors at Calcutta's Woodlands Nursing Home rate her chances of survival at 50-50.
Hundreds of well-wishers were keeping vigil outside. The clinic is as good as any in London, and some of India's best heart and lung specialists hovered by her bedside in the intensive care unit.
Mother Teresa was slipping in and out of consciousness, and it was not known whether she was aware of a visit by Jyoti Basu, the Communist Chief Minister of West Bengal, who has been both her adversary and reluctant admirer for decades. In Calcutta she and her nuns have been doing the work of caring for the poor and dispossessed, which glaringly underlines all that the Communist state government has failed to do.
Yet the gratitude Calcuttans feel for Mother Teresa for bringing light to the dark corners of the city's slums is mixed with some resentment. Bengalis are proud of their capital, and are pained that the world knows it only as irredeemably wretched, a theatre of horrors in which the nun, who is sure to be proposed for canonisation immediately after her death, performs her miracles.
Yet during her years in Calcutta, Mother Teresa has touched thousands of lives. She received "the call" to leave the security of the convent after she found a woman lying in a sewer too ill to fend off the ants and rats devouring her face. Since then she and her sisters have cared for a multitude of needy around the world, Christians, Muslims and Hindus alike.
She and her charity workers have fed half a million families, taught more than 20,000 slum children and treated more than 90,000 leprosy patients in hundreds of centres around the world including some in Britain. "God wants me to be poor and to love Him in the distressing disguise of the poorest of the poor," she once said.
Neither the Catholic Church nor her admirers want the flame of her kindness extinguished; yet Mother Teresa seems so strong in her faith and frail in body that she might prefer death to a life-support machine.