Some of the mourners waited overnight for a place and rushed into St Peter's Square when the metal barriers were opened at 6:00 am (0400 GMT).

The Argentine pontiff, who died on Monday aged 88, sought to steer the centuries-old Church into a more inclusive direction during his 12-year papacy -- attested by the 250,000 people who paid their respects before his coffin during its three days of lying in state.

"He was not just the Pope, he was what definition of being human is," said Andrea Ugalde, 39, who flew from Los Angeles to attend Saturday's mass.

US President Donald Trump is among more than 50 heads of state due for the ceremony set to start at 10:00 am (0800 GMT).

Crowds of 200,000 people are expected for the funeral, for which Italian and Vatican authorities have mounted a major security operation.

A no-fly zone is in place, fighter jets are on standby and snipers will be positioned on roofs surrounding the tiny city state.

Volunteers with walkie-talkies instructed people entering the plaza to slow down as they went through metal detectors. Within an hour after opening the plaza, seats were the public were mostly filled.

"We spent the whole night here in the car with the children," said Peruvian Gabriela Lazo, 41.

"We are very sorry for what happened to him because we carry a South American Pope in our hearts."

The funeral sets off the first of nine days of official Vatican mourning for Francis, who took over following the resignation of Pope Benedict XVI in 2013.

After the mourning, cardinals will gather for the conclave to elect a new pope to lead the world's 1.4 billion Catholics.

- Diplomatic gathering -

Many of Francis's reforms angered traditionalists, while his criticism of injustices, from the treatment of migrants to the damage wrought by global warming, riled many world leaders.

Yet the former archbishop of Buenos Aires's compassion and charisma earned him global affection and respect.

Trump's administration drew the pontiff's ire for its mass deportation of migrants, but the president arrived late Friday with his wife Melania to pay tribute to "a good man" who "loved the world".

Making the first foreign trip of his second term, Trump will face dozens of foreign leaders keen to bend his ear over a trade war he unleashed, among other subjects.

Ukrainian President Volodymyr Zelensky had hoped for a meeting with Trump after the funeral, but said Friday he may miss the event after a deadly Russian strike on Kyiv.

Trump's predecessor Joe Biden is also attending the funeral, as are presidents Javier Milei of Argentina, Luiz Inacio Lula da Silva of Brazil, France's Emmanuel Macron and Lebanon's Joseph Aoun.

More than a dozen royals including Britain's Prince William will also be present.

Israel -- angered by Francis's criticism of its conduct in Gaza -- is sending just its Holy See ambassador. China, which does not have formal relations with the Vatican, is not sending any representative at all.

- Simple tomb -

Francis, elected in 2013, died of a stroke and heart failure less than a month after he left hospital where he had battled pneumonia for five weeks.

He loved nothing more than being among his flock, taking selfies with the faithful and kissing babies, and made it his mission to visit the peripheries, rather than mainstream centres of Catholicism.

His last public act, the day before his death, was an Easter Sunday blessing of the entire world, ending his papacy as he had begun -- with an appeal to protect the "vulnerable, the marginalised, and migrants".

The Jesuit chose to be named after Saint Francis of Assisi, saying he wanted "a poor church for the poor", and eschewed fine robes and the papal palace.

Instead the Church's 266th pope lived at a Vatican guesthouse and chose to be interred in his favourite Rome church, Santa Maria Maggiore -- the first pontiff to be buried outside the Vatican walls in more than a century.

His send-off will be a grand affair, featuring some 224 cardinals and 750 bishops and priests alongside world dignatories.

But the humble pope asked to be put inside a single wooden coffin to be laid in a simple marble tomb.

After the funeral, the coffin will be taken to Santa Maria Maggiore via the Fori Imperiali -- where Rome's ancient temples lie -- and the Colosseum.

A group of "poor and needy" will greet the hearse on its arrival, the Vatican said.

- Refusal to judge -

Francis's admirers credit him with transforming perceptions of the Church and helping revive the faith following decades of clerical sex abuse scandals.

He was considered a radical by some for allowing divorced believers to receive communion, approving the baptism of transgender believers and blessings for same-sex couples, and refusing to judge gay Catholics.

But he also stuck with some centuries-old dogma, notably holding firm on the Church's opposition to abortion.

The first trip of Francis's 12-year papacy was to Lampedusa, an Italian island that became a haven for asylum seekers, and he visited Greece's Lesbos island, flying 12 refugees home with him.

Some of those refugees will be at his funeral.