Rainier's health "remained worrying," three of his doctors said in a statement. "Because of the fragility of his cardiac, respiratory and kidney functions, the vital prognosis remains reserved."
The statement indicated that specialists were withholding a prognosis, meaning they are unsure he will recover.
Dr. Jean-Charles Piette, chief of internal medicine at La Pitie Salpetriere hospital in Paris, was asked to examine Rainier, the statement said. He and other specialists decided the prince must continue his current course of treatment.
Piette examined Rainier on Thursday night, said palace spokesman Armand Deus.
Rainier was hospitalized more than two weeks ago with a chest infection. His health suddenly worsened on Tuesday, and he was brought to the intensive care unit of Monaco's seaside Cardio-Thoracic Center.
Boyan Christophorov, a professor of internal medicine at Paris V University, said the phrasing of Rainier's medical bulletin suggested that his doctors seriously fear for the prince's life. "The terms of these releases are carefully weighed. That the patient's vital prognostic is 'reserved' means that his life is seriously threatened," Christophorov told the Associated Press.
At a service Thursday evening at Monaco's Sainte Devote church, the Rev. Fabrice Gallo asked worshippers to pray for Rainier, and priests at Good Friday services in Monaco requested more prayers for Rainier, Deus said.
"Every hour we turn on the news, hoping to hear something positive," said Nathalie Ponsenard, a Monaco nursery school teacher. She pointed to the flag atop the hilltop royal palace.
"While it's still up, we know he's still alive," she said.
Some residents braced themselves for bad news.
"He's done so much for us, even for the young people," said Melanie Poisson, a 17-year-old high school student. "He's been prince for my whole life. It's hard to imagine Monaco without him."
The prince 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. 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.
The prince, who assumed the throne in 1949, is beloved in Monaco for having transformed a Mediterranean state smaller than New York's Central Park into a modern and elegant enclave for the rich. Rainier's movie-star wife, Grace Kelly, died in a car crash in 1982.
Rainier's heir, Crown Prince Albert, 47, and his two daughters, Princess Caroline, 48, and Princess Stephanie, 40, have been at his bedside since he was taken to intensive care.
Monaco changed its succession law in 2002 so that Albert can assume the throne even though he is unmarried with no children and has no descendants. Under the revised law, power eventually could be passed from Albert to his siblings, who both have children.