Monaco ? Prince Rainier III, who reshaped Monaco and worked to overcome its reputation as “a sunny place for shady people,” died Wednesday, leaving the throne to Prince Albert II, his only son with actress Grace Kelly.

In power for 56 of his 81 years, Rainier was Europe’s longest-reigning monarch and the only ruler many of Monaco’s 32,000 residents had ever known. A veritable father-figure, he dragged Monaco into the modern age while preserving much of the Mediterranean charm and royal trappings of his tiny principality.

Before age slowed him, Rainier poured his energies into public works, earning the name “the builder prince.” He put Monaco — which is smaller than New York’s Central Park — on the world map with his April 18, 1956, marriage to Kelly, who gave up Hollywood fame to become Princess Grace.

Albert, 47, has been groomed from birth to succeed Rainier. Multilingual, U.S. educated, and a five-time bobsledding Olympian, he was at his father’s bedside when Rainier died at a hospital overlooking Monaco’s yacht-filled main harbor.

Rainier had been treated there for the past month for heart, kidney and breathing problems. Albert took over the royal powers last week because of Rainier’s ill health.

Rainier’s funeral will take place April 15 at the 19th-century Monaco Cathedral where he and Princess Grace wed. He is expected to be buried alongside her.

At the traditional midday changing of the palace guards ceremony Wednesday, drums were covered with black cloth. The body of Rainier, whose family dynasty took power in 1297, was moved to his hilltop palace where it will in lie in state, the palace said.

Rainier’s doctors called Albert about 30 minutes beforehand to tell him the end was near, the palace said. The palace did not say whether Rainier’s daughters, Princesses Caroline and Stephanie, were with him when he died.

Christopher Le Vine, whose mother is Princess Grace’s last surviving sibling, said Albert and Caroline called to inform him of Rainier’s death.

Prince Rainier III of Monaco, left, and his son Prince Albert of Monaco leave Monaco's cathedral after attending a religious service in 2001. Rainier died Wednesday at age 81. Prince Albert succeeds him to the throne.

“They’re doing remarkably well under the circumstances,” he said.

He said he and other Philadelphia-area relatives will go to Monaco for Rainier’s funeral. He said the prince had a “unique sense of humor” but he expects Albert to make his own imprint on the French-speaking principality.

“It’s not something that he hasn’t anticipated over these many years. He will make his own space there,” Le Vine said.

Albert is unmarried and has no children — a source of consternation to Rainier, who worried about continuing the Grimaldi family line.

During Rainier’s reign, Monaco expanded its territory by 20 percent with land reclaimed from the sea — allowing the government to boast that Monaco was the only country to grow so much by peaceful means.

Rainier, who reportedly once considered Marilyn Monroe for his bride, met Kelly in 1955 when he was 31 and she was the 25-year-old star attraction of the Cannes Film Festival. She already had an Oscar from the 1954 film “The Country Girl,” one of only 11 movies she made.

In January 1956, they announced their engagement and were married in April. Ten months later they had the first of their three children, Caroline. Albert’s birth came the next year, on March 14. Stephanie was born Feb. 1, 1965.

Rainier never remarried after Grace’s death in a car accident on Sept. 14, 1982, and often cut a lonely figure in his latter years.