Crowd-control operations are under way, with the city offering tents and camping grounds for hundreds of thousands of faithful pouring in without lodgings.
Thousands of police reinforcements are arriving from across Italy, with the authorities unsure of what to expect. Most forecasts see 2 million faithful arriving, but some say it could be 4 million - potentially doubling the population of Rome.
"We've got no real forecast [of how many pilgrims are arriving] and we must update the plan depending on the situation," said Guido Bertolaso, coordinating Rome security during the funeral. "There will be critical moments."
The massive influx of pilgrims, many carrying sleeping bags, is a tribute to history's most travelled pope, who visited 129 countries and territories, from Chile to Kazakhstan, preaching peace and unity.
World leaders planning to attend the funeral range from US President George Bush to Syria's President Bashar al-Asad, making the service the most high profile in Vatican history.
Pope John Paul I's funeral in 1978 was attended by 105 VIPs, with the US delegation headed by President Jimmy Carter's wife, Rosalynn, instead of the president himself.
To protect the dignitaries, Rome will close its airspace on Thursday and Friday for an 8km radius.
The nearby Ciampino airport will be closed, and traffic at Fiumicino international airport will be reduced by nearly a third.
Al-Asad is one of many world
leaders due to attend the funeral
The air force said it would ready anti-aircraft missiles and deploy a Nato surveillance plane.
Italy took similar measures during the Group of Eight summit in Genoa in 2001.
But experience will not be enough to cope with the influx, set to dwarf the 250,000 visitors who filled St Peter's Square for Mother Teresa's beatification in 2003 - among the most popular events of John Pau IIl's 26-year pontificate.
The low cost of modern air travel promises to draw more last-minute pilgrims. Austrian Airlines on Tuesday was offering a special low-fare ticket to Rome, and faithful from the late pope's native Poland sought charter flights.
"We have to wait and see how many charter flights we can get. With every minute, there are more and more people coming asking how we can help them get to Rome," said Mariola Peknicka from Poland's Krakow diocese, where Karol Wojtyla was archbishop from 1964 until he became pope in 1978.
"With every minute there are more and more people coming asking how we can help them get to Rome"
Rome's infrastructure and crowd control operations are feeling the strain. A Ukrainian woman died of a heart attack over the weekend, and many other pilgrims have passed out in the massive crowds spilling from St Peter's Square.
Visitors fought over spots on Rome buses, and witnesses said pickpockets were robbing tourists on crowded underground trains, knowing busy police could do little to stop them.
"There were a whole group of them on the subway and one reached right into my jacket pocket, while I was looking at him," said Sergio Fernandez, a Brazilian television reporter.
"We told the police later, but they couldn't do anything."