Mother Teresa declared Saint by Pope Francis
"We may have some difficulty in calling her 'Saint' Teresa," the pontiff said. "Her holiness is so near to us, so tender and so fruitful that we continue to spontaneously call her Mother."
He added: "She made her voice heard before the powers of this world, so that they might recognize their guilt for the crime - the crimes! - of poverty they created."
The unscripted comments came at a canonisation mass attended by 100,000 pilgrims, including 13 heads of state or government and hundreds of sari-clad nuns from Teresa's order, the Missionaries of Charity.
Queen Sofia of Spain and some 1,500 homeless people also looked on as Francis described Teresa's work in the slums of the Indian metropolis 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."
The joyful celebratory atmosphere in the Vatican was mirrored in Kolkata, where candles and flowers were laid on Teresa's tomb at the headquarters of her order.
"It is a day of rejoicing, a day of gratitude and a day of many, many blessings," senior sister Mary Lysa said.
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.
She "ceaselessly proclaimed that the unborn are the weakest", he said.
The ceremony came on the eve of the 19th anniversary of Teresa's death in Kolkata, where she spent nearly four decades working in wretched slums.
With the 16th century basilica of St Peter's glinting in the late summer sun, Francis led a ritual mass that has barely changed for centuries.
Speaking in Latin, he declared "blessed Teresa of Calcutta (Kolkata) to be a Saint ... decreeing that she is to be venerated as such by the whole Church."
After the mass, the 79-year-old pontiff boarded an open-topped jeep and toured around St Peter's square and surrounding streets to a rapturous reception from tens of thousands of well-wishers.