The funeral Mass readings reflect Francis’ priorities and nod to John Paul II.

As the Mass began, the pope’s body was brought in a simple wooden coffin out of the basilica and placed before the mourners as a choir sang a psalm. Mourners continued streaming into the square as the robed clerics took their seats, with crowds around the square watching the service on large TV screens.

Among those assembled are President Trump; former President Joseph R. Biden Jr.; and President Javier Milei of Argentina, where Francis was born. A group of refugees and homeless people, two groups for whom Francis advocated throughout his 12-year papacy, also planned to attend, according to the charity St. Egidio.

Here is what else to know:

• Simplified ceremony: The changes Francis introduced for a papal funeral last year reflect his view of the pope as a humble pastor rather than a powerful figure, although the rites will still be on a grand scale.

• Francis’ burial: After the Mass, a vehicle will transport Francis’ wood coffin to St. Mary Major, a papal basilica in Rome that the pope loved. A group of “poor and needy” people will greet his body at the steps, the Vatican said. Then, he will be buried during a private ceremony in a tomb with a one-word inscription “Franciscus.”

• Choosing a successor: After the burial, the focus will turn in earnest to the election of the next pope by the College of Cardinals. Several names have surfaced as possible successors.

• Politics as backdrop: The solemn funeral ceremony is unfolding against a backdrop of geopolitical turmoil and war. Some of those attending, notably Mr. Trump and President Volodymyr Zelensky of Ukraine, have been directly at odds with each other. It was unclear whether Mr. Trump and Mr. Zelensky would meet.

The first reading, from the Acts of the Apostles 10: 34-43, is part of a pivotal moment in the early church when Peter, one of Jesus’ disciples and a man whom Catholics consider the first bishop of Rome, learns through a vision that the good news of the Gospel message is not just for the original followers but for a wider world.

“This is the universality of the gospel and a welcome to all,” said Gemma Simmonds, an author and senior research fellow at the Margaret Beaufort Institute of Theology in Cambridge, England.

“That is very much what Pope Francis wanted to emphasize,” added Ms. Simmonds, who is also a sister in the Congregation of Jesus, a Catholic religious order for women. “He was constantly talking about going beyond the conventional boundaries of the church and speaking to people that the church had originally excluded.”

The second reading, from St. Paul’s letter to the Philippians, 3:20–4:1, argues that a Christian’s true citizenship is not on Earth but in heaven. The passage explains death as a transformation of the human body, which it calls the “lowly body,” through Jesus, into what Paul calls the “heavenly body.”

The third Gospel reading, John 21: 15-19, recounts a conversation between Jesus and Peter after Jesus’ resurrection, in which the disciple is restored to faith and given the leadership role in the early church. Three times in the passage, Jesus asks Peter if he loves him. Jesus also asks Peter to “tend my sheep” and “feed my lambs.”

According to the biblical account, the conversation takes place after Peter betrayed Jesus in the run-up to his crucifixion by denying that he knew him. Experts said that in choosing that Gospel reading, Francis — as he had during his ministry — was making a deliberate reference to human vulnerability and the need for God’s grace.

“I see all of these readings as a traditional affirmation of the Christian message of forgiveness and hope and affirmation and the messages that Pope Francis emphasized in his papacy,” said Tina Beattie, a professor emerita of Catholic Studies at the University of Roehampton in London.

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Also known as plainsong or plainchant, this has been the official musical language of the church for over 1,000 years: austere, somber, unaccompanied and monophonic, meaning that everyone sings the same vocal line, as opposed to more complex polyphony.

“For truly solemn occasions, the best way to express the continuity of the church’s musical tradition is just to have simple chant,” James D. Wetzel, the director of music at the Church of St. Vincent Ferrer in Manhattan, said in an interview. “The church promotes the golden age of Renaissance polyphony, Mozart, Duruflé, modern settings. But pride of place is given to chant.”

So at Francis’ funeral, the main components of the Requiem Mass that anchor the service — like the opening Introit (“Requiem aeternam dona eis, Domine”), Kyrie, Sanctus and Agnus Dei — and sections including the litany of names of saints are being chanted in Latin. The Sistine Chapel Choir, the Pope’s personal choir, is the featured ensemble.

At some other state religious services, like the funeral of Queen Elizabeth II or the coronation of King Charles III, the music is a virtuosic mix of the familiar, obscure and new — meant to be awe-inspiringly different from everyday worship.

But the plainchant melodies at Francis’ funeral will be known to churchgoing Catholics. The music’s sober, unadorned style fits with the ways in which he simplified the elaborate papal funeral rites.

There will be occasions during the funeral for choral singing other than plainchant, but the Vatican, in keeping with its standard practice, has not identified the composers, or if any new music has been written for the occasion. The church has a tradition of priest-composers, including directors of the Sistine Chapel Choir, whose settings may have been selected. It is likely that the psalm “Sicut cervus” (“Like the deer that yearns for running streams”) will be performed in Palestrina’s luminous setting, one of the great examples of Renaissance polyphony.

By intention, however, there will be no obvious musical innovations.

“This is the time for the oldest traditions,” Mr. Wetzel said. “Even with Pope Francis, who was not afraid of surprises or upsetting the apple cart, the music sung at his funeral will be entirely Catholic, with a capital C.”

In the movie “Conclave,” which dramatized the death of a pope and the tensely political process of choosing his successor, the dean was named Cardinal Lawrence and portrayed by a principled yet questioning Ralph Fiennes.

Unlike in the movie, however, Cardinal Re will not run the conclave to select the next pope. He will not even attend, since only cardinals below the age of 80 can cast a ballot for the pope in the Sistine Chapel, though he will still play an important role in the run-up to the gathering.

There is sustained applause in the crowd as the pope’s coffin is borne slowly toward the ceremony.

That embrace of modesty is etched throughout his legacy.

He sought to overhaul, and at times rejected, papal pageantry and the formality long associated with the Roman Catholic Church. He chose his papal name from St. Francis of Assisi, who dedicated his life to the poor. He rode in a Ford Focus, wore basic black shoes and took meals in a communal cafeteria. For his living quarters, he opted for a guesthouse next to St. Peter’s Basilica in Vatican City instead of the more ornate papal apartments.

And he set to work on simplifying papal funerals.

In April 2024, Francis approved a liturgical book that included a new set of rules emphasizing that “the funeral of the Roman pontiff is that of a pastor and disciple of Christ and not of a powerful person of this world,” Archbishop Diego Ravelli, master of apostolic ceremonies, said when the book was published.

This week inside St. Peter’s, where tens of thousands of faithful came to pay their last respects, Francis’ body lay in a coffin that was placed low to the ground instead of on an elevated bier. Instead of nesting the coffin inside two additional coffins before burial, as was done for popes in the past, Francis is being laid to rest in one.

The departure from earlier papal burials is “totally in line with how Pope Francis led,” said Mathew Schmalz, a professor of religious studies at the College of the Holy Cross in Worcester, Mass. He added of Francis, “He wanted a papacy that was more reachable, that was down to earth and was able to engage with Catholics in their daily lives.”

Of course, for a pope, simplicity is relative. While some of the funerary pomp and ceremony has been scaled back, Francis was still the religious leader to over a billion Catholics, and some traditions that date back centuries will endure. His funeral is being attended by dignitaries from around the world, and crowds have gathered in and around St. Peter’s Square for the funeral.

Whereas many popes have been buried at St. Peter’s, Francis will be interred at the Papal Basilica of Santa Maria Maggiore, a 17th-century chapel dear to him in Rome. His tomb, according to his will, is inscribed with a single word: “Franciscus.”

He was accompanied by his wife, Melania, and a clutch of senior aides. But his predecessor, former President Joseph R. Biden Jr. and his wife, Jill, flew in on their own.

Considering that Mr. Trump had recently stripped Mr. Biden of his security clearances, and regularly denounces him, it is understandable that they did not share a ride to Rome. When former President Jimmy Carter died in December, Mr. Trump said he had passed away “a happy man” in the knowledge that “he wasn’t the worst. President Joe Biden was.”

Mr. Biden has often talked about his Roman Catholic upbringing. He saw Pope Francis briefly at a summit meeting in Italy last June, weeks before the disastrous debate with Mr. Trump that led to his decision to drop out of the presidential race. Mr. Biden had talked about returning to Rome to get the pope’s blessing just before leaving the White House, but canceled the trip because of the Los Angeles wildfires in January.

Asked on Air Force One on the flight to Rome on Friday evening whether he planned to talk with Mr. Biden at the funeral, Mr. Trump sounded surprised that his predecessor was attending. “It’s not high on my list,” he said. “It’s really not.”

The contrast with the 2005 papal funeral was stark. Mr. Bush was eager to ensure that Mr. Clinton was part of the full presidential party. Not only did the two men attend the funeral together, the former president sat in on the highly classified presidential daily brief on the flight over.

But the joined-at-the-hip act ended in the evening. Mr. Bush went to sleep early, around the time Mr. Clinton was just heading out. One night during the visit, Mr. Clinton was dining in a private room at a small, highly rated Italian restaurant when he realized that many of the White House reporters who had covered him as president were eating at a big table in the main dining room.

Naturally, he called us in to join him for dessert and espresso, and told stories until the early hours of the morning.

Vatican protocol offers a solution to the potential geopolitical awkwardness: the alphabet.

At Francis’ open-air funeral Mass in St. Peter’s Square, members of foreign delegations are divided into groups, like monarchs and heads of government, and will be seated in their group in alphabetical order based on their country’s name in French, according to a list released by the Vatican. Reigning monarchs go first, followed by heads of state, heads of government, royals and on down the line to include ministers and other dignitaries.

Only the heads of state of Italy and the pope’s native Argentina will get privileged seats. The delegation with the country’s president, Javier Milei, will sit closest to the square, the Vatican said, followed by Italy’s.

Some other recent high-profile ceremonies produced interesting seating combinations — such as the reopening of Notre-Dame Cathedral in Paris, when Mr. Trump sat next to President Emmanuel Macron of France, or former President Jimmy Carter’s funeral, when Mr. Trump was beside former President Barack Obama.

For the Vatican, all countries are equal before the alphabet. But that doesn’t mean there’s no chance of awkward moments.

According to the Vatican’s list, the president of the United States — les États-Unis in French — is likely to be between the leaders of Estonia and Finland, two countries that share a border with Russia and may look warily upon Mr. Trump’s courting of Moscow as he seeks to end the war in Ukraine.

The guest list the Vatican released, if followed for seating, could lead to geographically disparate pairings like those at some international summits. President Halla Tomasdottir of Iceland is supposed to be side be side with President William Ruto of Kenya. Leaders from Belize and Austria may also sit together as they say their last goodbyes to Francis.

Most streets leading to the Vatican were closed to regular traffic, and more than a dozen bus lines were diverted. Helicopters could be heard flying near St. Peter’s Square.

Some 3,000 volunteers would help those attending the funeral and support law enforcement agents, a spokesman for Italy’s Civil Protection Department said. Hundreds of emergency health workers are on hand to provide medical assistance.

The area is expected to be packed. Huge screens were set up at various points leading to the square to allow those further away to follow the funeral.

As is the case when there are large gatherings at St. Peter’s, like the Sunday Angelus prayer, entering the square requires passing through several security checks, including X-ray machines and metal detectors.

The Italian news media reported that snipers would be placed on rooftops during the proceedings, and specialized police units would patrol the Tiber River. Police forces were also stationed along the route from St. Peter’s to the Basilica of St. Marie Major, or Santa Maria Maggiore, where the pope will be buried after the funeral.