MONACO (AFP) - Residents of the tiny principality of Monaco on Thursday flocked to the royal palace to congratulate Prince Albert II and his wife Charlene on the birth of crown prince Jacques and his twin sister Gabriella.

Albert officially announced the birth of the twins in a video message to the nation, as well-wishers crowded the entrance of the palace where two framed documents proclaiming their arrival in French and English went up at midday.

"It is with great joy that I have the pleasure of announcing that Princess Charlene gave birth... to a boy and a girl named Jacques Honore Rainier, who will bear the title of crown prince, and Gabriella Therese Marie, princess and second in the line of succession," a beaming Albert said.

Red and white flags fluttered and the sky was filled with the sound of cannon shots and boat horns on Wednesday night as the Mediterranean principality welcomed the next head of the 700-year-old House of Grimaldi and his sister.

Gabriella was born at 5.04pm (12.04am Singapore time on Thursday) followed by Jacques two minutes later.

The palace said the newborns would be presented to the public from the palace balcony on Jan 7, which has also been declared a public holiday.

Claude Gauthier, 76, and his 73-year-old wife Josette were among the first to arrive at the palace on Thursday to sign a book of congratulations.

"We are happy and proud that our sovereigns have given life and ensured the continuity of the dynasty," Claude wrote out painstakingly with a black quill.

The modern monarchy has also set up an email address to receive congratulations from the public.

Albert, 56, and Charlene, 36, have also asked for donations rather than gifts that can be passed on to charities of their choice.

The babies replace Albert's sister Caroline as heirs to the throne, and will likely draw a line under years of rumours of the royal couple's supposedly rocky relationship.

The prince's late father, Rainier III, had rewritten the constitution when he became ill in 2002 so one of his daughters could inherit if his son - who had fathered two children out of wedlock - failed to produce a legitimate heir.

But Albert - whose late mother was Hollywood actress Grace Kelly - finally married former Olympic swimmer Charlene in 2011 - 11 years after the pair first met.

The birth of the twins at the hospital named after their grandmother Princess Grace was celebrated with 42 cannon shots, 21 for each child, fired from an old fort overlooking the sea.

The gender of the twins had been kept a secret during Charlene's pregnancy, even from their father who said he wanted to be surprised.

FIRST LEGITIMATE OFFSPRING

Albert, who was once considered one of the world's most eligible bachelors, succeeded his father Prince Rainier in 2005 at the age of 47.

He already had a daughter, Jazmin, 22, after a fling with former waitress Tamara Rotolo. He denied being her father for years before DNA tests proved otherwise when she was already teenager.

The prince also has a younger son, Alexandre Coste, 11, from an affair with Nicole Coste, a Togolese former Air France hostess.

Under Monaco's inheritance laws, neither of them have any claim to royal titles or to be considered as heirs to Albert because they were born outside of marriage.

They do however have legal rights to a share of his huge personal fortune, estimated by Forbes magazine to exceed US$1 billion (S$1.3 billion).

Monaco - the world's second smallest independent state - has 37,000 inhabitants with over 120 nationalities. Only about 8,000 who have proved their Monacan credentials are considered Prince Albert's subjects.

TUMULTUOUS PRIVATE LIFE

Zimbabwe-born Charlene tied the knot with Albert three years ago, despite rumours that their relationship was on the verge of collapse.

Media reports suggested she had attempted to flee Monaco just days before their wedding.

It was also widely reported that the couple spent at least part of their honeymoon in separate hotels. A steady stream of pictures of the princess looking gloomy continued to fuel reports she was depressed.

Charlene hails from a middle-class Zimbabwean family with German roots who relocated to South Africa when she was 11.

She swam for her adopted country in the 4 x 100m medley relay team at the Sydney Olympics in 2000.

Crown Prince Jacques will receive the title of Marquis of Baux, while his sister will be known as the Countess of Carlades.