The Basilica was open for most of the night, shutting its doors for only three hours between 2.30am and 5.30am on Friday.
The body of the 88-year-old pontiff, who died on Monday in his rooms at the Vatican's Santa Marta guesthouse after suffering a stroke, was brought to St Peter's in a solemn procession on Wednesday.
Since then, almost 130,000 people from all over the world have bid farewell to the pontiff, the Vatican said.
Queues on Friday morning after the basilica reopened for the day were stretching halfway down the main boulevard leading through Rome into the Vatican.
People were pressing forward slowly, some waiting hours, in order to have a few minutes inside to pay their respects to Francis.
Vatican officials plan to end viewings at 7pm on Friday before a formal rite to seal the late Pope's coffin.
"What surprised me was how determined he was to serve the church and love his people with all his energy, to the very end," Italian Cardinal Giovanni Battista Re - the ceremonial leader of the College of Cardinals and a retired Vatican official - told Italian daily la Repubblica in an interview published on Friday.
A conclave to choose a new pontiff is unlikely to start before May 6.
In the meantime, the world's Catholic cardinals have assumed temporary control of the 1.4-billion-member Roman Catholic Church.
Cardinals present in Rome are convening almost daily, primarily to discuss logistical matters, in what is called a "general congregation".
Some 113 of the world's 252 cardinals were present for the meeting on Thursday, the Vatican said, with dozens more expected to have arrived in Rome for Friday's gathering.
Francis's coffin will be sealed in a private ceremony on Friday evening led by eight Catholic cardinals, including a US prelate who has faced criticism over his handling of sexual abuse cases.
Among those also present will be the late pontiff's secretaries.
Rome is bracing before the arrival of hundreds of high-profile delegations attending Saturday's funeral, including US President Donald Trump, who will be flying into the Italian capital late on Friday.
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