Pope Leo XIV began post-surgery recovery on Wednesday after a tumour removed from his lung was found to be cancerous, plunging the Catholic Church into a period of uncertainty.
Part of the pope's right lung was removed in the operation and the leader of the world's 1.4-billion Catholics is awaiting the results of tests to determine if the cancer has spread.
"The pope passed a calm night," the Vatican said, without providing details.
The sporty 71-year-old, who was elected as the first American pope only last year, made the surprise announcement that he would be undergoing surgery after Sunday prayers.
He underwent the 90-minute operation at Rome's Gemelli hospital on Tuesday, after which the Vatican said he woke up normally.
"The hospital is doing everything it can to help him get better, so let's hope he recovers soon," student Chiara Ruvolo said outside the hospital north of the Vatican, where a banner read "Long live the pope!"
Experts said much would depend on how advanced the cancer was, and whether it had spread, but the Vatican has said it was detected early and was localised.
"When caught in early stages, then it has a very very good prognosis, which is what we're hoping and praying for with Pope Leo," American Lung Association medical spokesperson Juanita Mora said.
The Vatican has said it expects the pope's recovery to take around a week. All his audiences have been cancelled until October 20.
Leo is staying in a room on the 10th floor of the building, next to a papal suite with a private chapel that is currently under renovation.
Professor Federico Venuta, head of Thoracic Surgery at the Policlinico Umberto 1 hospital in Rome, said he expected Leo to be up and about immediately.
"Generally, on the first day, patients undergoing this type of procedure are mobilised immediately," he said.
"They have a follow-up X-ray, resume eating with a light breakfast, and then start physical therapy right away — the sooner they get out of bed, the better," he said.
Chicago-born Leo, a keen tennis player, had entered the hospital smiling and waving on Tuesday.
In the hospital chapel, dozens of faithful prayed for the pope's health during the operation.
The shock news of his illness has rattled the Catholic Church, according to Vatican watchers.
"The Church is now in a state of uncertainty. When Leo was elected everyone foresaw a long papacy," expert Marco Politi, who has spent decades covering the Vatican, said. — AFP