Tens of thousands have filed into St Peter's Basilica at the Vatican throughout the night and into the morning on the last day to bid farewell to Pope Francis ahead of his funeral on Saturday.

Long 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's open-topped coffin is placed on a dais.

The Basilica was open for most of the night, shutting its doors for only three hours between 2.30am (0030 GMT) and 5.30am.

The body of the 88-year-old pope, 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, about 150,000 people from all over the world have bid farewell to the pontiff, the Vatican said.

"It's a very strong feeling (to be here)," said Patricio Castriota, a visitor who, like the pope, is from Argentina.

"This farewell was very sad, but I thank God that I was able to see him".

"He's the only pope we've had who came from South America, a pope who had many good intentions for the Catholic Church," said Castriota.

Francis, who was pope since 2013, was the first pontiff from the Western hemisphere and was known for an unusually charming, and even humorous, demeanour.

His 12-year papacy was sometimes turbulent, with Francis seeking to overhaul a divided institution but battling with traditionalists who opposed his many changes.

"He humanised the church, without desacralising it," said Cardinal Francois-Xavier Bustillo, who leads the Church on the French island of Corsica.

Queues 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, ahead of a formal rite to seal the late pope's coffin. The Vatican said it would close access to the line to enter the basilica around 6pm.

A photo exhibition of Pope Francis in Peru pays tribute to his visit in 2018. – AP

A conclave to choose a new pontiff is unlikely to start before May 6.

Cardinals present in Rome are convening almost daily, primarily to discuss logistical matters, in what is called a "general congregation".

There were 149 of the world's 252 cardinals present for the meeting in the morning, the Vatican said, with dozens more expected to arrive through the rest of the day.

Francis's coffin will be sealed in a private ceremony on Friday evening led by eight Catholic cardinals. Among those also present will be the late pope's secretaries.

Rome is bracing ahead of the arrival of hundreds of high-profile delegations attending the funeral, including US President Donald Trump, who will be flying into the Italian capital late on Friday.

Authorities have started ramping up security ahead of the ceremony, with snipers on rooftops, drones watching from the sky and an army device readied to neutralise hostile flying objects.

The heart of Rome is expected to be closed to traffic on Saturday to allow a funeral motorcade carrying the pope's remains to make its way slowly to the Basilica of Santa Maria Maggiore, where Francis, in a break from tradition, asked to be buried instead of St Peter's Basilica.

Crowds are expected to gather along the route, which will pass by many of Rome's famed monuments, including the Colosseum.

The pope's 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.