Vatican City, Holy See — Pope Francis's funeral will be held on Saturday, the Vatican announced, as world leaders from US President Donald Trump to Ukraine's Volodymyr Zelensky said they would attend to honour the Catholic leader.

The Argentine pontiff, 88, died on Monday from a stroke, less than a month after returning home from five weeks in hospital battling double pneumonia.

His funeral, which is expected to draw huge crowds, will take place at 10:00 am (0800 GMT) on Saturday in the majestic Baroque plaza in front of St Peter's Basilica at the Vatican.

Francis's coffin — which he previously ordered should be of wood and zinc — will then be taken inside the church and from there to Rome's papal basilica of Santa Maria Maggiore for burial. 

The first so-called "general congregation" of cardinals took place on Tuesday morning, kicking off a centuries-old process that culminates in the election of a new pontiff within three weeks.

The Vatican published the first images Tuesday of the pontiff in his open coffin, wearing his red papal vestments, a mitre on his head and a rosary between his fingers.

The images came from a Monday evening service in the chapel of Casa Santa Marta, the Vatican residence where he lived during his 12-year papacy, and where he died.

Francis's coffin will be transferred to St Peter's Basilica on Wednesday at 9:00 am, to lie in state until Saturday's funeral.

In St Peter's Square, believers and tourists alike gathered to pay their respects, some bearing flowers or candles and with many lost in their thoughts.

"When a church is left without its head we are all a little disoriented," said Slovenian priest Bastian Dolinsek, who said Francis singlehandedly brought the Church "one step forward".

"In these times everyone is looking for a fixed point and there is no fixed point," he said.  

The Holy See's official media Vatican News, released details about Francis's last hours.

The pontiff started feeling ill Monday at around 5:30 am, less than 24 hours after he greeted a crowd of faithful in St Peter's Square on Easter Sunday.

Before falling into a coma, he waved to his personal nurse Massimiliano Strappetti from his bed, in what Vatican News described as "a gesture of farewell".

On Sunday, he had thanked Strappetti for encouraging him to take what would become his last tour of St Peter's Square in his popemobile.

"Thank you for bringing me back to the square," Francis was quoted as saying.

He was pronounced dead at 7:35 am on Monday.

"He did not suffer. It all happened quickly," Vatican News said, citing people who were with the pontiff in his final moments.

Week of mourning

Tributes have poured in from around the globe for Francis, a liberal reformer who took over following the resignation of German theologian Benedict XVI in 2013.

Argentina announced a week of national mourning while India began three days of state mourning on Tuesday -- a rare honour for a foreign religious leader in the world's most populous nation.

Trump was among the first to confirm he would attend the funeral, even after Francis criticised his migrant deportations.

Other expected guests include European Commission chief Ursula von der Leyen, France's Emmanuel Macron, Ukrainian President Zelensky and the Spanish royals.

Italy, which is preparing for a major security operation, declared five days of national mourning -- longer than the three days observed for pope John Paul II in 2005.

Cardinals of all ages are invited to the general congregations, the second of which will be held on Wednesday.

At the first meeting, attended by around 60 cardinals, lots were drawn to pick three people to help the camerlengo, Cardinal Kevin Farrell, run the Vatican's day-to-day affairs until a new pope is elected.

Only those under the age of 80 are eligible to vote in the conclave, which should begin no less than 15 days and no more than 20 after the death of the pope -- meaning between May 5 and 10.

Speculation is already growing about who will take over from Francis, born Jorge Bergoglio, who was the first pope from the Americas and the first Jesuit.

Cardinal Francois-Xavier Bustillo, the bishop of Ajaccio in Corsica, told AFP as he arrived in Rome that it would be an "intense" time.

Franciscus

Francis, who during his papacy wore plain robes and eschewed the luxury of his predecessors, has opted for a simple tomb, unadorned except for his name in Latin, Franciscus, according to his will released on Monday.

In choosing to be buried in Rome's Santa Maria Maggiore basilica, he will become the first pope in more than 100 years to be laid to rest outside the Vatican.

His death certificate released by the Vatican said Francis died of a stroke, causing a coma and "irreversible" heart failure.

Although Francis was ordered by doctors to spend two months resting after being discharged from Rome's Gemelli hospital on March 23, the pontiff could not resist numerous public appearances. 

"I saw on Easter Sunday that the pope was tired. He kept going right to the end, right to his last breath," Bustillo said.

Advocate for change

The hugely popular Francis was an energetic reformer who sought to open the Church to everyone, but his views also sparked fierce internal opposition. 

Francis advocated tirelessly for the defence of migrants, the environment, and social justice without deviating from the Church's positions on abortion or priestly celibacy. 

Outspoken and stubborn, Francis also sought to reform the governance of the Holy See, expand the role of women and lay people, and clean up the Vatican's murky finances. 

Faced with revelations of widespread child sex abuse in the Church, he lifted pontifical secrecy and forced religious and lay people to report cases to their superiors -- measures that victims' groups said did not go far enough.

(Source: Alice RITCHIE / Clément MELKI, AFP)