MONACO (AFP) - Prince Albert II of Monaco and his wife Charlene on Wednesday welcomed their twin babies to the world, as cannons roared and church bells tolled in the tiny Mediterranean principality.

Despite being born two minutes after his sister Gabriella, Jacques is next in line to head the House of Grimaldi, whose origins go back seven centuries before equality between the sexes.

Gabriella Therese Marie was born at 5.04pm (12.04am Singapore time) followed by Jacques Honore Rainier at 5.06pm, the palace said in a statement, adding that the newborns and their mother were "doing well".

The first children of the glamorous South African Princess Charlene replace Albert's sister Caroline as heirs to the throne.

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

Church bells also rang out for 15 minutes, followed by boat horns.

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.

Albert - whose late mother was the Hollywood superstar Grace Kelly - also said earlier that if the twins were a boy and a girl, it would be the boy who would succeed him.

According to tradition, an official birth announcement, signed by Albert, will be displayed at the entrance to the palace and the public will be invited to sign a book of congratulations.

Monegasques have also been encouraged to fly the principality's red-and-white flag from their homes until the day the twins are formally presented to the nation by the royal couple from the palace balcony.

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

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 a teenager.

The prince also has a younger son, Alexandre Coste, 11, from an affair with Nicole Coste, a 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 one billion dollars.

Charlene, 36, a Zimbabwe-born former Olympic swimmer, married 56-year-old Albert in 2011 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.

Crown Prince Jacques will receive the title of Marquis of Baux, while his sister, second in line to the throne, will have the title of Countess of Carlades.

Rather than send gifts, the new parents have asked well-wishers to make donations to charity.