Pope John Paul I, who died in 1978 after only 33 days as pontiff, moved closer to sainthood today with the Vatican still having to dismiss lingering conspiracy theories that he was a victim of foul play.

Pope Francis beatified his predecessor at a ceremony in St Peter's Square before tens of thousands of people. Beatification is the last step before sainthood in the Roman Catholic Church.

John Paul was known as 'The Smiling Pope' because of his meekness and simplicity.

"With a smile, Pope John Paul managed to communicate the goodness of the Lord," Pope Francis said in his homily, speaking as people huddled under umbrellas in a thunderstorm.

"How beautiful is a church with a happy, serene and smiling face, that never closes doors, never hardens hearts, never complains or harbours resentment, does not grow angry or impatient, does not look dour or suffer nostalgia for the past".

Born Albino Luciani into poverty in a northern Italian mountain village in 1912, he was ordained a priest in 1935, a bishop in 1958 and a cardinal in 1973.

He was elected pope on 26 August, 1978 following the death of Pope Paul VI, taking the name John Paul to honour his two immediate predecessors.

Two nuns of the papal household who heard no response to knocks on his door at 5.20am on 29 September to bring coffee found him dead in his bed. Doctors said he died of a heart attack and aides said he had complained of chest pains the day before but did not take them seriously.

Pope John Paul I waves to crowd in St. Peter's Square in 1978

Conflicting versions

At first the Vatican, uneasy saying two women had entered the pope's bed chambers, said he was found lifeless by a priest.

The Vatican corrected itself but the misstep sprouted conspiracy theories.

In 1984 'In God's Name - An Investigation into the Murder of Pope John Paul I', by British author David Yallop that argued the pope was poisoned by a cabal linked to a secret Masonic lodge, spent 15 weeks on the New York Times bestseller list.

The New York Times' own review of the book, however, ridiculed Yallop's investigative techniques and in 1987 another British author, John Cornwell, wrote a book called 'A Thief in the Night', meticulously dismantling conspiracy theories.

Although widely debunked, the idea of a pope being murdered in his bedroom in the 20th century irresistibly entered the collective consciousness and in the film The Godfather Part III, a pope named John Paul I was killed with poisoned tea.

"There is no truth to it at all," said Vatican Secretary of State Cardinal Pietro Parolin, when asked about the conspiracy theories on Italian television on Friday.

"It is a shame that this story, this noir novel, goes on. It was a natural death. There is no mystery about it," Cardinal Parolin said.

Pope Francis blesses a baby as he leaves at the end of a beatification mass of late Pope John Paul I

Italian journalist and author Stefania Falasca, who spent 10 years documenting John Paul's life and viewed his medical history, wrote several books about him. She called the conspiracy theories "publicity-driven garbage".

Ms Falasca, who was the deputy postulator, or promoter, of the sainthood cause, said he was being beatified not because of what he did as pope but the way he lived his life.

John Paul is attributed with the miracle healing of an 11-year-old Argentine girl who had a severe brain inflammation, epilepsy and suffered septic shock. Her parents prayed to him.

The Roman Catholic Church teaches that only God performs miracles, but that saints, who are believed to be with God in heaven, intercede on behalf of people who pray to them.

A second miracle will need to be verified for John Paul to be declared a saint.