Pope Leo XIV has been admitted to Rome's Gemelli Hospital for surgery to remove a lump on his lung, detected during what the Vatican has described as "routine checks".
It said the operation would take place on Tuesday afternoon.
The 71-year-old pontiff was met at the hospital by a large crowd, some of whom had been waiting outside for several hours to catch a glimpse of him.
They cheered greetings and well wishes as he passed, waving and smiling back through the lowered back window of his car.
He was then met by staff on the hospital steps. "I'm sorry," one of them commiserated, to which the Pope gave a smile and a shrug. "We're here for you," he was assured.
It was Leo XIV himself who on Sunday announced news of his operation to the world, speaking from the balcony in St Peter's Square after delivering the traditional Angelus blessing.
He said the nodule, or growth, on his lung was "contained" and had been found "at a very early stage", but that doctors had advised him to have it removed.
The Vatican said the Pope would be operated on by the Gemelli's thoracic surgery team and promised an update on his condition on Tuesday evening.
His schedule has already been cleared until 20 October to give him time to recover.
Nodules are quite common and not necessarily cancerous. But doctors say when a lump is removed it will usually be tested to check for any sign of the disease.
The Vatican has stressed that the pontiff, who is generally thought to be fit, had no symptoms of infection or illness.
Cardinal Kikuchi Tarcisio Isao of Tokyo wrote on social media that he'd had an audience with Pope Leo on Monday, and said the pontiff felt "nothing wrong" physically.
He said the Pope had decided to check in for surgery as soon as possible because he was due to travel to Latin America in early November, and was keen to stick to that plan.
The trip is particularly significant because it includes a visit to Peru, where the pontiff was a missionary and then a Bishop for many years.
Despite all the reassurances, the news has shaken Catholics all over the world.
"Am I worried? Yes, because I certainly wasn't expecting anything like this," Vatican Secretary of State Cardinal Parolin told journalists on Tuesday.
He agreed that the news was like a "bolt from the blue", but said the cardinals had been struck by the Pope's "serenity" and were confident in his recovery.
Outside Gemelli Hospital, which has treated several popes and is home to a large statue of John Paul II, a few Catholics lingered after the pontiff was admitted.
Carla had come specially, wanting to be close.
"He seems calm, so we are calm too - even though illness is always unpleasant," she told the BBC. "We will wait here and pray."
"I was in St Peter's Square when he made the announcement, and I felt really awful," said Elsa, who was also present. "I was really taken aback," she said, explaining that when Pope Leo was elected she had thought he was a younger, healthier pontiff.
"It turns out he's vulnerable too. I really hope it's something superficial, that can be contained, like he told us."