Pope Francis has declared Mother Teresa of Calcutta, known as the "saint of the gutters" during her life, a saint, just 19 years after her death.

Applause erupted in St Peter's Square at the Vatican even before Francis had finished pronouncing the rite of canonisation at the start of the Mass for the nun on Sunday, with the attendance of tens of thousands of pilgrims.

Pilgrims streamed into the square from early morning before a service to honour the nun and Nobel peace laureate who worked among the world's poorest in the slums of the Indian city now known as Kolkata.

"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.

The legacy of Mother Teresa

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."

Role model

Millions of Catholics revere the nun as a model of compassion, and more than 100,000 people were expected at the mid-morning ceremony in front of St Peter's Basilica, decked out with a canvas of the late nun in her trademark blue-hemmed white robes.

Mother Teresa's legacy complements Pope Francis' vision of a humble Church that strives to serve the poor, and the festivities are a highlight of his Holy Year of Mercy, which runs until November 8.

Canonisation mass is attended by tens of thousands of Catholic pilgrims [Reuters]

Al Jazeera's Laurence Lee, reporting from the Vatican, said that speeding up the canonisation of Teresa helped Pope Francis' campaign for social justice. 

"Before Pope Francis took over the job, the Catholic Church was in a real mess. There were scandals affiliated with the church and corruption, the resignation of Pope Benedict and the loss of direction of the Church," he said.

"Pope Francis is trying to see a resetting and a new sense of direction revolved around social change and helping the poor. And speeding up the canonisation of Teresa helps this goal." 

Source: Al Jazeera News and agencies