Prince Albert II assumed the throne of the principality of Monaco this morning on a day of festivities in the Riviera to which his illegitimate son was not invited.

Prince Albert became His Serene Highness of Monaco at an ornate coronation service in the cathedral where his father, Rainier III, was buried three months ago. The mass will be followed by a garden party this afternoon and a concert and fireworks display this evening, on a day declared a holiday in the principality.

The voices of the Little Singers of Monaco, trumpets and soprano solos rang through the white stone cathedral to mark the Prince's formal assumption of royal powers.

Hundreds of Monegasques attended the coronation, which was given by Monsignor Bernard Barsi, the archbishop of Monaco. Prince Albert, 47, takes the throne after 56 years of his father's rule and becomes the first bachelor ruler of the principality. Monaco's constitution was reformed to make way for his accession in 2002.

Two notable absences from the celebrations were the Prince's former lover, Nicole Coste, and his 22-month-old illegitimate son, Alexandre. After remaining silent during the three months of mourning that followed his father's death, Prince Albert acknowledged his son for the first time last week.

The Prince told French television that he would support his son, but that the child would never bear the royal Grimaldi name that has governed Monaco since 1297.

The Prince also warned that there may be more revelations about his private life to come in the future, of a similar nature. "We will answer when the moment comes," he said.

Today's installation will be followed by a further ceremony on November 19, which heads of state are expected to attend. But messages recognising Prince Albert's accession started to arrive this morning. President Chirac invited the Prince to Paris in September and said: "Your reign inaugurates the coming of new relations between our two countries."

Prince Albert is the only son of his father, Rainier, and Grace Kelly, the beautiful actress who gave up Hollywood to become a Princess. Princess Grace was killed in a car crash in 1982.

The Prince has promised to marry in the future and to modernise the government of Monaco, which has been accused of allowing the wealthy principality to become a centre for money laundering and gambling.

"We must absolutely free ourselves of this equation that Monaco equals laundering," the Prince told French television last night.

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