Carla Bruni may be the First Lady of France, but she had no chance of becoming the “first diva” at an exuberant Nelson Mandela 91st birthday concert in New York. Bruni’s first public performance since marrying Nicolas Sarkozy, who was in the audience, put her on stage at Radio City Music Hall along with Gloria Gaynor, Aretha Franklin, Queen Latifah, Alicia Keys, L’il Kim and Cyndi Lauper, plus the male acts will.i.am, Wyclef Jean, Josh Groban, Jesse McCartney and Stevie Wonder.

Gaynor kicked off the tribute with her 1978 classic I Will Survive, Franklin wowed the crowd with her virtuoso arpeggios and Queen Latifah introduced her mother and announced: “She taught me all women are queens, and should be treated as such.”

It was undoubtedly brave for Bruni to line up with such music royalty. Whoopi Goldberg, acting as master of ceremonies, said there were “more stars than you ever saw in astronomy class”.

Bruni is said to have broken her vow not to perform while her husband is in office because the concert benefitted Mandela’s Aids charity 46664 — named after his prisoner number on Robben Island. Mr Mandela’s brother died of Aids.

Her performance was pretty, but bloodless and immobile — like her face. Dressed in a swish black trouser suit and strumming her guitar, she looked like a soixante huitard hipster. She sang two duets with her collaborator Dave Stewart, the first her most popular song, Quelqu’un m’a dit (Someone Told Me), a folksy ballad with unmistakably existentialist lyrics, the second the Sixties anthem Blowin’ in the Wind.

Although it was perhaps unprecedented for a First Lady of a great power to sing at a pop concert, France appears to have developed etiquette for the event. Bruni did not hug — much less kiss — Stewart, either before or after their set, and barely swayed as she performed her songs. Her husband, sitting half way back in the glorious golden clam-shell that is Radio City Music Hall, clapped but did not budge from his seat when others rose.

As soon as she finished her performance, the couple left to catch a plane for France — drawing several previously unidentified secret service agents out of surrounding seats. Her early departure meant that she missed the grand finale, when all the artists danced rapturously together as Stevie Wonder sang Happy Birthday to Mr Mandela.

Stewart, who has been writing unpublished songs to Bruni’s French lyrics in recent years, has compared Bruni’s voice to Dylan’s — apparently a compliment. Where Dylan has a whiney edge, however, Bruni’s rasping is more like pillow talk. Quelqu’un m’a dit is an endearing solo in her breathy, feminine tones. But it left Stewart with little role but to ooh and aah in French during the chorus. It was hardly a duet.

Their choice of Blowin’ in the Wind was problematic for other reasons. It seemed ironic to hear Bob Dylan’s protest song sung by a former Italian supermodel and heiress who is the wife of a conservative president (remembering she also fled the Red Brigades as a child).

Next to the unbridled passion of the African and African-American performers, Bruni’s rendition seemed insipid — at least until Stewart amped up his electric guitar at the end.

The pair got a standing ovation from the politically charged audience, however, for the lines: “How many years can some people exist / Before they’re allowed to be free?”

Stewart has said that he and Bruni may soon release the songs they have co-written, though Bruni said that they were still not finished. She bridled at a question about whether she planned to restart her musical career, which has been on hold since she married Sarko. “I never stopped my music career,” she said.

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