The day after Easter, St. Peter's Square was packed.
Caramba Camarra, a Gambian volunteer with Opera Romana Pelligrinaggi, the Vatican-run pilgrimage agency, said he had never seen so many people lined up to visit the basilica.
"It's amazing! The line curves like a serpent, filling the whole square," he said. "It looks like the crowds at Mecca."
What's known as "the Francis effect" referring to the hugely popular current pope has boosted Rome tourism: March alone brought more than 2 million visitors. And the Vatican and Rome are preparing for that many visitors, and perhaps more, for Sunday's canonizations of two of Francis' predecessors, John Paul II and John XXIII.
Vatican organizers say it will be a sober affair, in line with Pope Francis' no-frills style. Nevertheless, an onslaught of millions of pilgrims and tourists is expected, testing an ancient and normally chaotic city.
Tour manager Giorgio Sansa is worried.
"In a city which is nearly 2,800 years old and with a population some say 2.5, some say 4 million if 3 million people are coming in a city like this, an old city, there will problems without doubts, we cannot do anything about it," he said.
For comparison: The Vatican spokesman estimated that St. Peter's Square and the wide avenue leading into it can hold about 250,000 people. The crowd for Easter Sunday this year was estimated at 150,000.
Rome Mayor Ignazio Marino is convinced Rome can handle the big numbers expected this weekend.
"We are prepared to distribute almost 4 million plastic bottles of water, we have a massive plan to host almost 2,000 buses coming from all over Europe, and we will have shuttles moving back and forward in order not to have 2,000 huge buses circulating downtown in Rome," Marino said.
Some 2,000 police officers will be on the streets at any given time, says the mayor. More than 2,500 civil protection department volunteers will help with crowd control. Specially trained medical teams will staff 13 first aid stations. And a thousand chemical toilets have been set up near the Vatican and key tourist areas.
Rome's two subway lines and some buses will run nonstop, and 17 big screens will enable visitors to watch the ceremony throughout the city.
The total cost is estimated at $11 million, which will be partially covered by the national government. After all, the mayor says, this is not just a local Roman event.
"We are talking about an event I would classify as a global event," Marino said. "We will probably have 2 billion all over the planet who will watch Rome on that day through TV, radio and Internet."
Precisely because so many eyes will be on Rome this Sunday, the city has undertaken a massive operation to clean up its tattered image. The city will be fixing potholes and taking down unlicensed billboards.
Sansa, the tour manager, says it's embarrassing to explain to visitors why in Rome there's also a great ugliness.
"They ask me about the graffiti, the garbage, and especially the state of the roads," he said, "the public transport, the traffic, the pollution, the dirt."
The mayor is convinced Rome will show its best and reap benefits from the mega-event. But he's well aware of the many things that could go wrong.
"The pope told me, 'You will dance on April 27,' using an expression that in Italy means you will have troubles on that day," Marino said. "I hope that I will have to dance just a little."
Not all Romans are happy about the event. Taxi drivers are furious that much of the city will be pedestrian-only. Many pilgrims will be day-trippers, spending little on food and nothing on lodgings.
And many Romans who can, will get out of town.