Carla Bruni sings for Nelson Mandela

In the company of Gloria Gaynor, Aretha Franklin and Queen Latifah - 'queens' of disco, soul and rap respectively - Carla Bruni bravely took to the stage of New York's Radio City Music Hall on Saturday night for a birthday concert in honour of Nelson Mandela. She appeared a little stiff and formal according to first reports, but she got through two duets with Dave Stewart, the former Eurythmics guitarists, while her husband Nicolas Sarkozy applauded from the stalls.

Bruni, the former model who has been singing and playing accoustic guitar for some years, had vowed not to perform in public while her husband is President of France. She broke her rule because the concert benefited various Aids charities - the disease that killed her brother Virginio three years ago.

She sang two duets with Stewart - her best-known French song, a folk ballad called Quelqu'un m'a dit (Someone told me), and then the classic Bob Dylan Sixties protest number, Blowin' in the Wind.

Dressed in a black trouser suit, Bruni was clearly having to be on best 'first lady' behaviour. She barely moved to the music and abstained from hugs and kisses with co-performers.

As Lizzy Davies wrote for the Guardian: "She spoke in her trademark husky drawl and sported the centre-parting of a 70s folk singer, but there was still something unmistakably sober about Carla Bruni-Sarkozy."

James Bone, reporting for the Times, called her performance "pretty but bloodless and immobile - rather like her face".

She slipped out before the finale to fly back to France with her husband - missing the chance to join the other performers on stage for Stevie Wonder's Happy Birthday to Mandela. The former South African president himself was unable to attend due to poor health.