Cardinals were due to meet on Tuesday to plan Pope Francis' funeral, which leaders from around the world will attend ahead of a conclave next month to elect a new head of the Roman Catholic Church.

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 for double pneumonia. But he returned to his Vatican home almost a month ago and had seemed to be recovering, appearing in St. Peter's Square on Easter Sunday.

His sudden death 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 so they cannot be used by anyone else.

"We want to thank the Lord for the gifts he has given to the whole Church with the apostolic ministry of Pope Francis, a pilgrim of hope," said Cardinal Mauro Gambetti, who led prayers in St. Peter's Square on Monday evening.

All cardinals currently in Rome have been invited to gather in the Vatican at 9 a.m (0700 GMT), where they were expected to make funeral plans.

U.S. President Donald Trump, who clashed repeatedly with the pope about immigration, said he and his wife would fly to Rome for the event. Among other heads of state set to attend were Javier Milei, president of Francis' native Argentina.

The Vatican has said it expects the ceremony to take place sometime between Friday and Sunday. 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 Basilica.

The gathering of cardinals will also 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 normally 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 participate in the highly secretive ballot which can stretch over days.

At present there is no clear frontrunner to succeed Francis.

PROGRESSIVE

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 LGBTQ community.

Francis appointed nearly 80% 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.

Many of the cardinals are little known outside their own countries and they will have a chance to get to know one another at meetings known as General Congregations that take place in the days before a conclave starts and where a profile of the qualities needed for the next pope will take shape.

The Vatican said late 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.

His body could be moved to St. Peter's Basilica as early as Wednesday morning for the faithful to visit, the Vatican said.