Crown Prince Albert, Rainier's only son and his heir, was seen entering the Cardio-Thoracic Center as were his two sisters, Princesses Caroline and Stephanie. Rainier's movie-star wife, Grace Kelly, died in a car crash in 1982.
The 81-year-old prince's condition — he remained on a respirator after suffering heart and kidney failure — cast a somber mood over this glitzy Riviera principality.
"Monaco became Monaco thanks to him. I think of him like a second grandfather," said Sandrine Negre, 22, out strolling with friends near the hospital. "This really hurts me."
A medical bulletin released Wednesday by the royal palace described Rainier's condition as "stable." The prince's entourage said Thursday that there had been no change, and no new official medical bulletin was released.
The prince had been hospitalized more than two weeks ago with a chest infection. After a marked improvement, the prince's health suddenly worsened on Tuesday.
He was transferred to intensive care unit after developing a sudden respiratory infection "with cardiac and kidney failure" and breathing trouble that made a respirator "indispensable," according to the medical bulletin.
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.
"Monaco prays for Prince Rainier," read the main headline of Monaco Matin newspaper, with a photo of people lighting candles for him at church.
"This country is Prince Rainier," said Patricia Vermeulen, 53, a retired schoolteacher. "This fabulous adventure that is Monaco — he created it."
Rainier has been in and out of the hospital recently. He has a history of heart problems and has lately been plagued by recurring ailments linked to his respiratory tract.
Infections can bring on congestive heart failure, which can lower blood pressure and ultimately lead to kidney failure. Heart failure also depresses the respiratory system, making breathing difficult.
Doctors often use respirators, and dialysis machines, to lighten the workload of the body while it heals from an infection. However, respiratory infections in the elderly can be deadly.