Cher and the Jonas Brothers Dazzle the French Riviera

On Thursday night, roads were closed for miles around the Hotel Du Cap-Eden-Roc on the French Riviera, helping to keep out paparazzi and eager fans.

On the vast estate overlooking the crystal blue water of the Mediterranean, over 800 actors, performers, models, and royals — including Demi Moore, Michelle Yeoh and Magic Johnson — walked a red carpet lined with photographers for the annual amfAR gala, an event started by the actress Elizabeth Taylor in 1993 to raise money for AIDS research.

The event has often attracted marquee acts (Diana Ross and Mariah Carey performed in past years), and this year, Cher, who at 78 is still releasing new music, was tapped to perform.

Lindsey Wixson, a model who has worked for brands including Chanel and Prada, summed up the mood before the performance: “I’m freaking out a little,” she said. “I’m a huge fan, and she is such a legend.”

Held on the second Thursday of the film festival at the Hotel Du Cap-Eden-Roc in Cap d’Antibes in the French Riviera, where rooms start at $1,800 a night, the gala serves as an unofficial closing party for the Cannes Film Festival, which lures some of the most famous faces in the world every May. (Elle Fanning, Meryl Streep, Emma Stone, Joe Alwyn and Greta Gerwig were all spotted during this year’s festival.)

For some, it had been a busy week.

Earlier that day, the actress Diane Kruger was at a private villa overlooking Cannes for the ceremony for the inaugural Humann Prize, an award for environmental and social activism. She presented an award to Golshifteh Farahani, the Iranian-French actress.

Ms. Kruger didn’t have much time to party, she said, because her film, “The Shrouds,” premiered on Monday.

“The most fun I had was at my after-party,” she said. “It was at Picasso’s old house, and it was pretty cool to get a tour and see his art and hang out in his gardens.”

Emma Thynn, the Marchioness of Bath, an English socialite, spent the previous day at a lunch for emerging actors co-hosted by Vanity Fair and IHG Hotels & Resorts at the Carlton Beach Club, a chic beachside restaurant in Cannes that was overflowing with rose and seafood.

Kelly Rowland, the singer and actress, said she had actually spent the festival watching films. “Being able to watch films in French,” she noted, “it’s kind of a vibe.”

Guests entering the amfAR party on Thursday night walked through the marbled lobby of the hotel, down a grand white staircase and out to the back garden, where they were greeted with champagne.

Diplo, the D.J., started hunting for a drink the moment he arrived. “I’m excited about the first margarita I’m about to have,” he said. “If they aren’t serving them, I can teach them how to make one.”

Jason Oppenheim, a star of the reality show “Selling Sunset,” which follows his Los Angeles real estate company and its team, was admiring the elaborate gowns. “That woman over there with the black feathers coming out of her dress is stunning,” he said. “I’ve stepped on five trains already.”

People crowded around Sarah Ferguson, the Duchess of York, who, like King Charles III and Catherine, Princess of Wales, is battling cancer. She gave an update on her health during the reception.

“I’m doing really well, actually. I’m really pleased to be standing here,” she said in an interview, adding it would be “wonderful” if the other royals were able to attend next year.

A little after 8 p.m., guests were ushered into a vast tent made to look like an old movie theater.

Ms. Moore welcomed the crowd, the chattering of which was so persistent that she had to chide guests to pay attention: “I hear a lot of noise.”

Over a dinner of truffled zucchini flowers, beef brulee and profiteroles, organizers auctioned off items including a seven-day trip to Antarctica and the opportunity to be a walk-on actor in an upcoming season of “Emily in Paris.”

Carine Roitfeld, the editor of CR Fashion Book, curated a collection of outfits shown by models for the auction, each created by a different designer and inspired by fairy tales. Givenchy designed a green turtleneck maxi gown made to look like the Wicked Witch of the West (the model wore a witch’s hat) while Loewe designed an open slit dress with red sequins like the Queen of Hearts from “Alice in Wonderland.” (The gala raised $16 million for AIDS research.)

The Jonas Brothers then took the stage and performed “Cake by the Ocean,” which got the crowd, including Ms. Moore, dancing.

Then, just after midnight, Cher made her entrance, dancing down the catwalk in a blonde wig, purple cargo pants and a blazer decorated with rhinestones to sing “Walking in Memphis” and “Waterloo.” Then she changed into a black wig, a sparkly leotard and knee-high boots for her hits “Believe” and “If I Could Turn Back Time.” (Her outfit was a version of the costume she originally wore 35 years ago in the music video for “If I Could Turn Back Time.”)

As Cher sang, Ms. Moore danced wildly alongside Ms. Yeoh and Andie MacDowell. Ms. Roitfeld beamed up at the stage. Nick and Joe Jonas sang along to the songs.

After much applause, the lights came on and guests were shuffled into the after-party by the hotel pool.

There, hotel staff members offered valuable supplies: slippers, phone chargers, sliders and espresso martinis.