Huge queues snaked around St Peter's Square and the surrounding roads, before being funnelled through the heart of the basilica in a single column leading to the central altar, where Francis' open-topped coffin is placed on a dais.
The viewing started on Wednesday and the Vatican had expected to close the doors at midnight but ended up letting the faithful in until 5.30am, before reopening again at 7am.
"After three and a half hours in line, we had sore feet and tired legs. But we did see Francis and the experience, while exhausting, was spiritually affirming," said Richard Lamb of New Jersey in the USA, visiting with his wife Peggy, both devout Catholics.
"We walked with groups from Italy, Spain, England, Germany, Brazil, Australia, Poland, the US, the Philippines ... it was a long, sublime experience," he said.
Francis died aged 88 on Monday morning in his rooms at the Vatican's Santa Marta guesthouse, ending an often turbulent 12-year reign in which he sought to overhaul a divided institution, battling with traditionalists who opposed his many changes.
"He was a wonderful pope," said Rome resident Alessandra Caccamo, as she queued outside the Vatican.
"I'm going to miss him so much, because it's like I've lost a piece of me."
By 7pm more than 90,000 people had paid their respects, Vatican officials said.
There was no stopping for a lingering view once the faithful got to the foot of the coffin, which was flanked by Swiss Guards in red-plumed helmets, as ushers swiftly moved people on.
Only clerics and special guests were allowed to sit for a while on pews set up on either side of the casket, including players from the city's AS Roma football team, all wearing matching dark suits.
More than 130 delegations, with 50 heads of state including US President Donald Trump and 10 reigning monarchs, are expected in St Peter's Square for Saturday's funeral, the Vatican said, with millions more expected to watch on television across the globe.
Authorities in Rome started ramping up security ahead of the ceremony.
A police source told Reuters snipers would be placed on rooftops, with drones watching from the sky and an army device readied to neutralise hostile flying objects.
A conclave to choose a new pontiff cannot start before May 6.
Cardinals gathering in Rome have assumed temporary control of the Church, each taking an oath of secrecy not to reveal their discussions about a future pontiff.
This does not stop the powerful prelates from talking to reporters about their vision for the 1.4-billion member Church.
"A chapter in the Church's history has been closed," Cardinal Gerhard Ludwig Mueller told Italian daily La Repubblica in an interview published on Thursday.
Mueller is one of the 135 cardinals eligible to vote in the secret conclave.
A former head of the Vatican's doctrine office, Mueller is known for his traditionalist views and often clashed with Francis.
The German cardinal said that there was "unanimous appreciation" for the late pope's work on migrants and the poor.
The Vatican said on Thursday a group of "poor and needy" had been invited to welcome the pope's casket to Rome's St Mary Major - a church 4km from St Peter's where, in a break from tradition, Francis wanted to be buried.
"The poor have a privileged place in the heart of God," the Vatican said.
The heart of Rome is expected to be closed to traffic on Saturday to allow the funeral motorcade to make its way slowly to St Mary Major, giving Romans a chance to say a final farewell.
His tomb will be in a niche in a side aisle of the basilica, with just the word "Franciscus," his name in Latin, engraved on the marble.
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