Cardinals were meeting Tuesday to set the date for Pope Francis's funeral, as world leaders from US President Donald Trump to Ukraine's Volodymyr Zelensky said they would attend to honour the Catholic leader.
The first so-called "general congregation", called following the death on Monday of Francis, 88, signals the start of a centuries-old tradition that culminates in the election by cardinals of a new pontiff within three weeks.
The first images of the Argentine pontiff in his open coffin were published by the Vatican on Tuesday.
The pope was seen during a service Monday evening in the chapel of Casa Santa Marta, the Vatican residence where he lived during his 12-year papacy, and where he died.
Francis was wearing his red papal vestments, a mitre on his head and had a rosary between his fingers.
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.
His home country, Argentina, prepared for 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.
Heads of state and royalty are expected for his funeral, due to be held at St Peter's Basilica, with Trump and France's Emmanuel Macron the first to announce they would attend.
On Tuesday, a source at the Ukrainian presidency told AFP that Zelensky, too, would come to Rome.
The funeral should be held between the fourth and sixth days after the pope's death, according to the Apostolic Constitution -- so between Friday and Sunday this week.
But the details will be decided by the cardinals, whose first of a series of "general congregations" was scheduled for 9:00 am (0700 GMT) Tuesday.
Cardinals of all ages are invited to the congregations, although only those under the age of 80 are eligible to vote for a new pope in the conclave.
The conclave should begin no less than 15 and no more than 20 days after the death of the pope.
Simple tomb
The pope's body was moved into the Santa Marta chapel on Monday evening, and his apartment formally sealed, the Vatican said.
His remains are expected to be transferred to St. Peter's Basilica starting Wednesday to lie in state.
Francis, who 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 Monday.
He will be buried in Rome's Santa Maria Maggiore basilica, becoming 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.
He had been discharged from Rome's Gemelli hospital on March 23 and ordered to spend at least two months resting.
But Francis, who never took a holiday and delighted in being among his flock, made numerous public appearances in recent days.
He appeared exhausted on Sunday during the Easter celebrations, but nevertheless greeted the crowds in his popemobile in St Peter's Square.
Argentine football great Lionel Messi hailed his compatriot -- himself a huge fan of the beautiful game -- for "making the world a better place".
Eyes of God
On Monday evening, thousands of faithful, some bringing flowers or candles, flocked to St. Peter's Square at sunset to pray for Francis.
He "tried to get people to understand it doesn't matter your sexual orientation, your race, it doesn't matter in the eyes of God", Mateo Rey, 22, a Mexican student, told AFP.
"I think that's the closest to what Jesus intended."
Born Jorge Bergoglio, Francis was the first pope from the Americas and the first Jesuit to lead the worldwide Catholic Church.
An energetic reformer, he sought to open the Church to everyone and was hugely popular -- but his views also sparked fierce internal opposition.
In 12 years as pope, Francis advocated tirelessly for the defence of migrants, the environment, and social justice without questioning the Church's positions on abortion or priestly celibacy.
Outspoken and stubborn, Francis also sought to reform the governance of the Holy See and expand the role of women and lay people, and to 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.
However, victims' groups said he did not go far enough.
