Throughout Monaco, Rainier's subjects braced for what they feared could be the final days of a man many revere as a father figure.
The 81-year-old head of Europe's longest ruling royal family, was spending his third day in intensive care at a hospital with a view of his hilltop palace.
"Monaco became Monaco thanks to him. I think of him like a second grandfather," said Sandrine Negre, 22. "This really hurts me."
A medical bulletin from the royal palace yesterday described the ailing prince's condition as "stable."
The prince, whose film star wife, Grace Kelly, died in a 1982 car crash, was admitted to hospital more than two weeks ago with a chest infection. After a marked improvement, his health suddenly worsened.
Rainier was transferred to the intensive care unit at Monaco's Cardio-Thoracic Centre on Tuesday after developing a sudden respiratory infection "with cardiac and kidney failure," his palace said.
"Breathing difficulties made the installation of artificial respiration indispensable," a spokesman said.
Rainier's heir, Crown Prince Albert, 47, returned from abroad on Tuesday to visit his father, as did Princess Caroline, 48, and Princess Stephanie, 40.
Outside the hospital, daily life continued as normal in the tiny Riviera principality wedged between the mountains and the Mediterranean. But the prince's fragile state was close to many. Some watched the palace and its flagpole for signs.
Rainier, who assumed the throne in 1949, is beloved in Monaco for having transformed a state smaller than New York's Central Park into a modern and elegant magnet for jet setters.
"This country is Prince Rainier," said Patricia Vermeulen, a 53-year-old retired school teacher who lives near the royal palace. "This fabulous adventure that is Monaco, he created it."
In 2002, the constitution was revised to allow the unmarried Albert to succeed his father, despite his lack of heirs.
Rainier's two daughters for years have been the focus of paparazzi, who fed off their rocky love lives. Stephanie, known as the wild child, had three children out of wedlock, then married a circus acrobat.