Pope John Paul I, the ‘smiling pope’ who reigned for just 33 days before his death in September 1978, was beatified in a Mass led by Pope Francis on Sunday.

Pope John Paul I (born Albino Luciani, October 17, 1912 - 28 September 1978) was head of the Catholic Church and sovereign of the Vatican City from August 26, 1978 to his death 33 days later.

His reign is among the shortest in papal history, resulting in the most recent year of three popes and the first to occur since 1605.

John Paul I remains the most recent Italian-born pope, the last in a succession of such popes that started with Clement VII in 1523.

At the beatification Mass, Francis again quashed speculation that the newly beatified pontiff's death had not been a natural one, from a heart attack.

He paid tribute to his predecessor's "mildness" and "humility".

"With his smile he succeeded in transmitting the Lord's goodness," said Francis.

The mass was attended in a heavily rainy St Peter's Square by over 25,000 faithful, including those from the late pope's old diocese of Belluno-Feltre, Vittorio Veneto and Venice.

Thousands of faithful gathered in St Peter's Square for the mass. (Photo: AAP)

The 33-day pope, Francis said, "became the image of a Church with a happy face, a serene face, a smiling face, a Church that never closes its doors, that does not harden hearts, that does not complain and does not harbour resentment, which is not angry, is not impatient, does not present itself in a stern way, and does not suffer from nostalgia for the past".

Francis unveiled a picture of John Paul I, the first in the Vatican painted by a Chinese artist, the hyper-realist Yan Zhang.

The day of devotion to the new blessed will be August 26, the day of his election in 1978.

The beatification required the recognition by the Vatican of a miracle wrought by the late pope. In John Paul I's case it was the healing of an 11-year-old Argentinian girl who had been at death's door in 2011.

She had been on the verge of succumbing to multiple epileptic fits, pneumonia and septic shock when the local priest, who was a devout admirer of John Paul I, started leading prayer sessions that brought about her full recovery, said the Dicastery for the causes of Saints.

The smiling pope is now aiming to ascent the last rung of the ladder to sainthood, for which at least another miracle is needed.