World leaders and rank-and-file Catholic faithful have bidden farewell to Pope Francis in a funeral that highlighted his concern for people on the peripheries and reflected his wish to be remembered as a simple pastor.

Although presidents and princes attended the Mass in St Peter’s Square, prisoners and migrants welcomed Francis’ coffin at his final resting place in a basilica across town.

The Argentine Pope, who reigned for 12 years, died at the age of 88 on Monday after suffering a stroke.

According to Vatican estimates, about 250,000 people flocked to the funeral Mass at the Vatican and 150,000 more lined the motorcade route through downtown Rome to witness the first funeral procession for a pontiff in a century.

They clapped and cheered “Papa Francesco” as his simple wooden coffin travelled aboard a modified popemobile to St Mary Major Basilica, 6km away.

US President Donald Trump and former president Joe Biden, Ukrainian President Volodymyr Zelensky, United Nations Secretary-General António Guterres and UK Prime Minister Keir Starmer joined Prince William and continental European royals leading more than 160 official delegations.

As bells tolled, the pallbearers brought the coffin past several dozen migrants, prisoners and homeless people holding white roses outside the basilica.

Once inside, the pallbearers stopped in front of an icon of the Virgin Mary that Francis loved.

Four children deposited the roses at the foot of the altar before cardinals performed the burial rite at his tomb in a nearby niche.

The Pope’s casket is carried in St Peter’s Square. Photo: AAP

In front of hundreds of world leaders, Italian Cardinal Giovanni Battista Re called for care for migrants, an end to wars, and action on global climate change — Francis’s favourite political themes.

He repeated one of the Pope’s strongest criticisms of US President Donald Trump, with Trump himself present in the crowd, by calling to “build bridges, not walls”.

It was a criticism Francis levied against Trump in 2016, when the real estate mogul was making his first run for president.

Trump, the pope said, was “not Christian” because of his views on immigration.

“A person who thinks only about building walls, wherever they may be, and not building bridges, is not Christian,” said Francis at the time.

“This is not in the gospel.”

Trump responded then, “For a religious leader to question a person’s faith is disgraceful.”

More recently, the Pope had called Trump’s immigration crackdown in his second term a “disgrace”.

Prince William represented the King at the funeral. Photo: AAP

The sermon on Saturday contained a strong political message for the national leaders and a strong internal message to the world’s Catholic cardinals.

To the about 135 Catholic cardinals who will soon be tasked with entering a secret conclave to choose the next pope, it was also a possible roadmap for how they should start their deliberations.

Re also cited a major document written by Francis in 2015 on climate change, which was intended to influence discussions at the 2016 Paris climate conference, and visits by the pope to the Mediterranean islands of Lampedusa and Lesbos, where he met migrants in detention camps.

Despite Francis’ focus on the powerless, the powerful were out in force at his funeral.

Argentine President Javier Milei had pride of place given Francis’ nationality, even if the two did not particularly get along and the Pope alienated some in his homeland by never returning there.

Trump sat with the rows of foreign dignitaries on one side of Francis’ coffin in the vast St Peter’s Square.

On the other side sat cardinals who will pick Francis’ successor at a conclave next month.

Francis repeatedly called for an end to conflict during his papacy.

His funeral provided an opportunity for Trump, who is pushing for a deal to end Russia’s war with Ukraine, to meet Ukrainian President Volodymyr Zelensky inside St Peter’s Basilica.

A sea of clergymen among the thousands who gathered in Rome. Photo: AAP

Applause rang out as Francis’ coffin, inlaid with a large cross, was brought out of the basilica and into the sun-filled square by 14 white-gloved pallbearers at the start of the Mass.

The crowds clapped loudly again at the end of the service when the ushers picked up the casket and tilted it slightly so more people could see.

Aerial views of the Vatican showed a patchwork of colours – black from the dark garb of the world’s leaders, red from the vestments of 250 cardinals, the purple worn by some of the 400 bishops and the white worn by 4000 attending priests.

Francis, who shunned much of the pomp and privilege of the papacy, had asked to be buried at St Mary Major Basilica rather than in St Peter’s — the first time a pope had been laid to rest outside the Vatican in more than a century.

-with AP