Crowds stretched down the main boulevard leading through Rome into the Vatican, pressing forward slowly in warm spring sunshine as people took their turn to see the late pontiff in his open coffin.
The body of the 88-year-old Pope, who died two days ago in his rooms at the Vatican's Santa Marta guesthouse after suffering a stroke, was brought to St Peter's in a solemn procession earlier on Wednesday.
Red-hatted cardinals, bishops, candle-carrying friars and helmeted Swiss Guards walked slowly into the vast square in front of the basilica as a choir chanted psalms and prayers in Latin while a bell gently tolled.
As the coffin crossed St Peter's Square, a crowd of several thousand broke into repeated applause - a traditional Italian sign of respect at such events.
"He's like a member of the family. Somebody very close to our hearts, somebody who made the Church very accessible to everybody and inclusive to everybody," said Rachel Mckay, a pilgrim from the United Kingdom.
Vatican officials helped 14 white-gloved, black-suited pallbearers carry the coffin up a stone incline, before the procession passed through St Peter's giant bronze doors and into the hushed interior of the ornate, cavernous church.
Francis, who had only recently left hospital after five weeks being treated for double pneumonia, last appeared in public on Sunday, when he delighted onlookers gathered to celebrate Easter by being driven around the packed square in his white open-topped popemobile.
As the formal ceremony to bring the Pope's body to the basilica was ending, Sister Genevieve Jeanningros, a nun from the Roman seaside town of Ostia, stood by his casket and cried.
Crowds were guided through the central nave of the basilica to come forward and offer respects.
Francis was dressed in red vestments, his hands clasped together holding a rosary, and a white mitre on his head.
Alex Lenrtz, a pilgrim from the United States who was among the first public mourners to stand in line, said he felt a mixture of emotions.
"It's just such a surreal (feeling) ... and seeing the body and just remembering everything that he stood for is very important."
Francis' body will lie in state in St Peter's Basilica until Friday evening.
The Vatican had originally planned to close Wednesday's visits at midnight but said it may extend the timeframe, due to a "high flow" of pilgrims.
In the first eight and a half hours of viewing, 19,430 people filed in front of the coffin, with thousands more waiting patiently in a line that snaked around the security barriers across the cobbled esplanade outside the church.
Saturday's funeral will draw heads of state and government from around the world.
Leaders from the US, Italy, France, Germany, UK, Ukraine, Brazil, European Union institutions and Francis' home country of Argentina have confirmed their presence, among many others.
A conclave to choose the new pontiff is not expected to start before May 6.
The cardinals now gathering in Rome will decide the date following what are often prolonged discussions.
There is no clear frontrunner to succeed Francis although UK bookmakers have singled out Luis Antonio Tagle, a reformer from the Philippines, and Pietro Parolin, from Italy, as early favourites.
During the ceremony earlier on Wednesday, Tagle and Parolin stood together in the basilica, flanked by about 80 other cardinals, as the wooden coffin was laid on a dais.
The body was placed in front of the altar, built on the spot where St Peter, the first pope, is believed to have been buried after dying as a martyr in the reign of Emperor Nero (54-68 AD).
Francis shunned much of the great pomp and ceremony traditionally associated with the role of head of the world's 1.4 billion Roman Catholics.
He clashed repeatedly with traditionalists, who saw him as overly progressive and too accommodating to minority groups such as the LGBTQ community.
In electing a new pope, cardinals will have to consider whether to complete Francis' promised reform of the Church, making more room for women in senior positions and being more amenable to an evolving society, or opt for retrenchment.
Only 135 cardinals are eligible to participate in the secretive conclave, 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.
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