Francis, 88, died unexpectedly on Monday after suffering a stroke and cardiac arrest, the Vatican said, ending an often turbulent reign in which he repeatedly clashed with traditionalists and championed the poor and marginalised.
The pontiff spent five weeks in hospital earlier this year suffering from double pneumonia.
But he returned to the Vatican almost a month ago and had seemed to be recovering, appearing in St Peter's Square on Easter Sunday.
The Vatican on Tuesday released photographs of Francis dressed in his vestments and laid in a wooden coffin in the chapel of the Santa Marta residence, where he lived during his 12-year papacy.
Swiss Guards stand on either side of the casket.
His body will be taken into the adjacent St Peter's Basilica on Wednesday at 9am (5pm AEST) in a procession led by cardinals, allowing the faithful to pay their last respects to the first Latin American pope.
His funeral service will be held in St Peter's Square, in the shadow of the Basilica, on Saturday at 10am.
US President Donald Trump, who clashed repeatedly with the Pope about immigration, said he and his wife would fly to Rome for the service.
Among other heads of state set to attend are Javier Milei, president of Francis' native Argentina, Brazilian President Luiz Inacio Lula da Silva and Ukrainian President Volodymyr Zelenskiy, according to a source in his office.
In a break from tradition, Francis confirmed in his final testament released on Monday that he wished to be buried in Rome's Basilica of Saint Mary Major and not St Peter's, where many of his predecessors were laid to rest.
Francis's sudden death has set in motion ancient rituals, as the 1.4-billion-member church started the transition from one pope to another, including the breaking of the pope's "Fisherman's Ring" and lead seal, used in his lifetime to seal documents, so they cannot be used by anyone else.
All cardinals in Rome were summoned to a meeting on Tuesday to decide on the sequencing of events in the coming days and review the day-to-day running of the church in the period before a new pope is elected.
A conclave to choose a new pope usually takes place 15 to 20 days after the death of a pontiff, meaning it should not start before May 6.
Some 135 cardinals are eligible to take part in the secretive ballot, which can stretch over days before white smoke pouring from the chimney of the Sistine Chapel tells the world that a new pope has been picked.
There is no clear frontrunner to succeed Francis.
Pope Francis inherited a church in disarray and worked hard to overhaul the Vatican's central administration, root out corruption and, after a slow start, confront the scourge of child abuse within the ranks of the priesthood.
He often clashed with conservatives, nostalgic for a traditional past, who saw Francis as overly liberal and too accommodating to minority groups, such as the LGBTQI community.
Francis appointed nearly 80 per cent of the cardinal electors scattered across the world who will choose the next pope, increasing, but not guaranteeing, the possibility that his successor will continue his progressive policies.
The Vatican said late on Monday that staff and officials within the Holy See could immediately start to pay their respects before the pope's body at the Santa Marta residence, where Francis set up home in 2013, shunning the grand, apostolic palace his predecessors had lived in.
