MONACO - Prince Rainier III, whose marriage to American film star Grace Kelly brought elegance and glamour to one of Europe's oldest dynasties, died today at the hospital treating him for heart, kidney and breathing problems. He was 81.

Monaco's royal palace announced Rainier's death nearly a month after he was first admitted with a lung infection to a heart and chest clinic that overlooks Monaco's glittering, yacht-filled harbor.

He had been Europe's longest-reigning monarch.

"Each of us feels like an orphan because the principality has been marked by his imprint over the 56 years" he ruled, said Patrick Leclercq, Monaco's head of government.

Prince Albert, 47, Rainier's heir, was at his side when he died at 6:35 a.m. Rainier's doctors had called Albert around 6 a.m. to tell him the end was near, the palace said. Rainier died from the heart, kidney and lung problems that caused his hospitalization, the palace said.

Rainier's 1956 marriage to Kelly - a year after he met the then 25-year-old at the Cannes Film Festival - captivated the world. Princess Grace died in a car accident in 1982.

Albert, their only son, becomes Monaco's de facto ruler until a formal investiture ceremony expected after a mourning period. He took over the royal powers - but not the throne - from his father last week after a royal commission decided Rainier was too sick to rule.

The unmarried Albert inherits a French-speaking principality no larger than New York's Central Park but world-renowned for its casino and the annual Monte Carlo Grand Prix. It's nestled on the Mediterranean Coast between Italy and the French Riviera.

He is unmarried and has no children. Monaco changed its succession law in 2002 to allow power to pass from a reigning prince who has no descendants to his siblings. Albert has two sisters, Princess Caroline and Princess Stephanie, both of whom have children.

Rainier, who assumed the throne on May 9, 1949, had to endure the tragedy of his famous wife's death and relentless scandals - including international criticism of the principality's tax laws - that plagued the final two decades of his rule.

The leader of Europe's longest-ruling royal family, the Grimaldis, Rainier suffered recurring health problems in recent years. The silver-haired, portly prince underwent heart surgery in 1999 and had two operations the following year.

Recurrent chest infections put him in the hospital on numerous occasions. Most recently, he was hospitalized March 7. He was placed in intensive care two weeks later with heart and kidney failure and connected to a respirator.

Tributes flooded in from around Europe. French President Jacques Chirac hailed the prince's "courage and tenacity" in the face of his failing health. German President Horst Koehler said he "fulfilled his duties as head of state with remarkable willpower until the end."

Britain's Queen Elizabeth II sent a message of condolence to the family. Rainier's death means the queen, who acceded to the throne in 1952, becomes the longest-serving monarch in Europe.

In 1993, Monaco gained the political recognition Rainier sought for his principality with its entry into the United Nations.