On a night when celebrities can let loose on their fashion choices, pop princesses and Hollywood actress took it up a notch in the avant-garde department at New York's Met Gala on Monday.

The 2017 Metropolitan Museum of Art's Costume Institute ball was themed "Rei Kawakubo/Comme des Garçons: Art of the In-Between," an homage to highly structured creations of the Japanese designer and her avant-garde label.

Pop singer Katy Perry, a co-chair of the ball, lived up to the theme's challenge with a custom Maison Margiela "Artisanal" ensemble by John Galliano, an imperial red tulle dress and a veil embroidered with the word "Witness."

Eric Wilson, fashion news director at InStyle, described this year's theme as "extremely challenging" compared to years past and agreed that Perry's outfit was courageously on point.

"I think she's without a doubt the hit of the night so far," Wilson said. "It's very brave to wear something that disguises who you are so much on the red carpet."

Rihanna was no slouch either, embodying Kawakubo’s penchant for dramatic silhouettes in a custom Comme des Garçons floral dress with cutouts and oversized ruffles. The singer earned the most mentions on social media, said Kellan Terry, an analyst at the social media monitoring company Brandwatch.

"Happy" singer Pharrell Williams in jeans and leather jacket, and star couple Tom Brady and Gisele Bundchen in matching silver helped fashion maven and Vogue editor-in-chief Anna Wintour host the annual fundraising gala, which is sometimes referred to as "the Oscars of the East Coast."

Reality TV star Kim Kardashian went against the theme, sporting a white dress by Vivienne Westwood with no clutch, no jewelry and no Kanye West, her fashion-loving husband. Sister Kendall Jenner dared to be bare, wearing a sheer slip that left little to the imagination, both in front and back.

Donning a beige Ralph Lauren trench coat gown paired with big silver hoop earrings, actress Priyanka Chopra dropped jaws and turned heads. Tennis star Serena Williams also served up drama with a baby bump under her green Atelier Versace dress.

Actress Zoe Kravitz went with a strapless pink and black Oscar de la Renta gown with an off-the-shoulder train, receiving much praise on social media, where the hashtag #MetGala was top trending.

Wilson said an added plus to Kravitz's dress is that it "looks comfortable."

"Some of these pieces are gorgeous on the red carpet," Wilson said. "But I'm assuming that a lot of these celebrities are going to be changing once they get through that door and sit down to dinner."