A health bulletin signed by three doctors appeared grim, saying the state of health of the 81-year-old Rainier head of Europe's longest-ruling dynasty "remained worrisome."
The prince was suffering heart, kidney and breathing problems and the doctors withheld a prognosis, suggesting the outcome was uncertain.
"Because of the fragility of his cardiac, respiratory and kidney functions, the vital prognosis remains reserved," the statement said.
The medical team at the principality's Card-Thoracic Centre called in a specialist from Paris because of "the gravity of the situation," the latest medical bulletin said.
Dr Jean-Charles Pieta, chief of internal medicine at Paris' La Pitta Salpetriere hospital, examined Rainier, said palace spokesman Armand Deus. The team decided to continue the current treatment.
Priests asked worshippers at Good Friday services in Monaco to pray for Rainier, said Deus. Rainier has ruled the tiny Riviera nation smaller than London's Hyde Park since 1949, carrying on the Grimaldi dynasty, which is more than 700 years old.
"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 on top of the hilltop royal palace.
"While it's still up, we know he's still alive," she said.
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.
Boyan Christophorov, a professor of internal medicine at Paris University, said the phrasing of Rainier's medical bulletin suggested that his doctors 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," he said.
Rainier is like a father figure to many here and some residents were bracing themselves for what would be a new era.
"He's done so much for us, even for the young people," said Melanie Poisson, a 17-year-old student. "He's been prince for my whole life. It's hard to imagine Monaco without him."
The prince is beloved in Monaco, and has left his mark on his Mediterranean principality. He has transformed Monaco into an elegant enclave for the rich. Rainier's Hollywood-star wife, Grace Kelly, died in a car crash in 1982.
Rainier has been in and out of hospital recently. He has a history of heart problems and has recently suffered recurring respiratory tract problems.
Respirators are often used to lighten the workload of the body while it heals from an infection. However, respiratory infections in the elderly can be deadly.
Prince Albert, who is unmarried and childless, would succeed Rainier.