MONACO— Prince Albert II of Monaco formally became the ruler of this tiny Riviera principality Tuesday in a celebration that began with a solemn Mass at the cathedral where his father, Rainier III, was buried three months ago after a reign of 56 years.

His accession to the throne is a two-step event with another ceremony expected to draw heads of state planned for Nov. 19.

Wearing a dark suit and sky blue tie, a grave-looking Albert assisted at the Mass celebrated by the archbishop of Monaco, Monsignor Bernard Barsi. Albert was joined by his sisters, Princesses Caroline and Stephanie, and other members of the royal family,

Although the day began with an organ playing inside the white stone cathedral, it was scheduled to end with fireworks and a festive ball under the stars, to the music of Pink Floyd and U2.

The Little Singers of Monaco, the Monaco Philharmonic Orchestra, trumpets and soprano solos added to scene at the church.

The 47-year-old bachelor prince is the only son of Prince Rainier and actress Grace Kelly, who gave up Hollywood for the life of a princess before dying in a car crash in 1982.

Shops were closed and banners of red and white, the national colors, decorated the palace and windows around Monaco, a rocky Mediterranean enclave next to Nice, France.

Hundreds packed the stone cathedral near the royal palace.

“I remember Albert’s birth. We’ve seen him grow up,” Christine Rousguisto, 79, said before the Mass began.

Albert, often portrayed as shy and retiring, has the confidence of Monaco’s citizenry.

“Of course, he’s ready for this. He has everything to be a wonderful prince,” said Rousguisto.

At the close of the 75-minute service, Albert donned his sunglasses and walked with his sisters to the nearby palace for a garden party.

International recognition of Albert’s reign started arriving in Monaco with President Jacques Chirac of France inviting him to Paris in September.

“Your reign inaugurates the coming of new relations between our two countries,” Chirac said.

Secretive about his personal life, Albert has been more forthcoming since his father’s death. On the day Monaco’s official mourning period ended last week, he confirmed news reports that he fathered a child out of wedlock with an African-born flight attendant.

In a last-minute surprise before formally embracing the role of His Serene Highness, Albert told France’s TF1 television Monday night that there may be others waiting to step forward with paternity claims.

Asked to elaborate, he refused, saying only, “We will answer when the moment comes.”

In publicly acknowledging he is the father of a nearly 2-year-old child named Alexandre, Albert said he wanted to face up to his responsibilities. His son, he said, will want for nothing, although he will never take the throne and won’t bear the Grimaldi family name.

Monaco’s constitution was revised in 2002 to circumvent the problem his bachelor status created for the line of succession. The change allows power to pass from a reigning prince who has no descendants to his siblings, but forbids illegitimate children from acceding to the throne.

“He’s a modern prince,” said pharmacist Marie-Paule Velay, wearing a pale yellow hat. “He acted honorably,” she said, pointing out that he recognized the child soon after his birth.

The prince, whose bachelorhood caused consternation to his father, has said he plans to marry one day and father legitimate heirs eligible for the throne _ but only when he is ready.

In the TF1 interview, Albert said he wants to clean up the image of Monaco _ often called a sunny place for shady people _ during his reign.

A playground for the rich, Monaco is studded with banks and casinos, and has come under fire for money laundering.

“We must absolutely free ourselves of this equation that Monaco equals laundering,” Albert said.