Bruni-Sarkozy, a singer and former model, was joined on the front row at the Dior show by French actress Marion Cotillard, US socialite Olivia Palermo and Korean actress Claudia Kim.
The ex-first lady's 59-year-old husband, Nicolas Sarkozy, last week announced he would be seeking the leadership of the centre-right UMP party.
That was seen by many as a first step towards him making a run to regain the presidency in 2017 elections.
Dior artistic director Raf Simons presented his spring/summer 2015 collection in a courtyard at the Louvre museum on day four of the Paris ready-to-wear shows.
In a wearable collection, he sent out long, elegant coats, printed silk jumpsuits and embroidered satin ribbon dresses.
"The challenge was to bring the attitude of contemporary reality to something very historical; bringing easiness to something that could be perceived as theatrical. It is the attitude that matters," he said in show notes.
The show took place in the Cour Carree, the oldest courtyard at the Louvre Palace in Paris.
Nine days of ready-to-wear fashion shows run until Wednesday with the highlight due to be Saturday's show by French designer Jean Paul Gaultier, his last before he stops ready-to-wear to concentrate on haute couture.