The star-filled extravaganza that drew crowds and halted traffic was held on a 60-meter floating podium.
Uninvited guests clambered around barriers to get a free glimpse at Game of Thrones' Nikolaj Coster-Waldau and actress Eva Longoria who took turns as models, alongside Andie MacDowell who talked ageism to AP.
Here are some of the day's highlights:
L'OREAL GOES IN-SEINE
The clothes were designed by some of the great houses of Paris fashion including Balmain, AMI, Off-White, Giambattista Valli, Elie Saab.
Yet this fashion show, much like L'Oreal's display on the Champs-Elysees last year, was always more about the show than the fashion.
A giant floating board was this season's runway — flanked by hundreds of champagne-sipping VIP guests on the river bank and others peering out from the deck of a specially-hired boat.
Drones, meanwhile, buzzed overhead to stream the action via social media to 30 different countries, and traffic along the Seine was halted for the duration of the spectacle.
The display began to cheers as a speedboat that splashed down the Seine docked some glamorous freight: out stepped model Doutzen Kroes.
L'Oreal ambassadors then flooded the runway.
British signer Cheryl appeared in provocative thigh-high boots and a one-shoulder split-leg minidress with reflective paillettes that sparkled in the blazing sun.
Elle Fanning smiled sweetly as she walked in a pastel shoulderless embroidered gown and bright red heels.
Meanwhile, American actress Aja Naomi King made her L'Oreal modeling debut in a draped pink number.
But the king and queen of the show had to be Nikolaj Coster-Waldau and Eva Longoria.
The "Desperate Housewives" star stepped out in a regal, layered gunmetal mini-gown — with 2-meter-long train.
Coster-Waldau, meanwhile, was all smiles but looked slightly nervous to be there modeling to the crowds in a long-tailored coat and white shoes.
"I was surprised to be doing a catwalk. I never thought I would see the day — neither did my children!" Coster-Waldau said.
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ANDIE MACDOWELL TALKS AGEISM
Ahead of the L'Oreal show, Andie MacDowell, 60, spoke to AP about ageism and her longevity in being a L'Oreal ambassador, a post she'd held for some three decades, and counting.
"I think the timing right now is really fiery as far as acceptance. And ageism is part of that acceptance," the American actress said.
"I have to say you have to give L'Oréal credit for being one of the first people to take on all ages and to take on mature people like Jane Fonda, Helen Mirren and Diane Keaton ... and keeping me," she added, humbly. (Fonda and Mirren modeled in last year's Champs-Elysees show.)
MacDowell praised the Paris cosmetics giant for being a trail-blazer when it came to "recognizing that there is no expiration date on beauty."
In "how we treat women as they get older, I think it's important to be inclusive and also have a deep respect for them," MacDowell added.
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VALENTINO DELIGHTS
"A work in substraction," so said the house, was the spirit behind Pierpaolo Piccioli's accomplished — and pared down — display for Valentino.
Cactuses and cleanly shaped plants lined the foot of the runway, presumably in reference to the clean lines and minimalist styles that opened the show.
Deceptively simple black looks began the collection: a shoulderless baggy jumpsuit with cape and Elizabethan-style sleeves and a gown with an exaggerated peplum hem.
Their beauty lay in the subtlety of detail.
An unstructured minidress with giant flounce looked beautifully off-kilter as it hung delicately from the model's shoulder, as if it could fall off at any given point.
White looks then came, and were, alongside black, a dominant theme — speaking to the ubiquitous spring-summer trend.
Artistry was plentiful in some of these white looks: gowns with delicately-pressed pleats that seemed to fan around the belly button.
But Valentino is a couture house at heart, and despite this being a ready-to-wear show, the work of the "petites mains," or seamstresses of the age-old atelier, was on display.
An oversize, veiny wicker hat composed of billowing feather possessed a delicate organic feel, and had guests understandably reaching for their cameras.
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FREIDA PINTO FINDS TIME TO STEP OUT
Indian actress Freida Pinto rocked a beautifully tailored menswear jacket look on the Valentino front row — the first time she's been seen at one of the couture house's displays.
"I'm super excited. This is really my style. It makes me feel really comfortable," she said.
"This is my first ever Valentino show... We've been trying for some time to make it work but with my schedule it's been hard," she added.
Pinto, who shot to fame with "Slumdog Millionaire," blamed her busy schedule on several "exciting" films she's starring in.
It includes the British-American fantasy adventure "Mowgli," based on the Rudyard Kipling fable set in India, in which she plays Messua, who decides to adopt the wild Mowgli, believing that he is their long-lost son Nathoo. It also features Christian Bale and Cate Blanchett.
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GIVENCHY'S SLIGHT GEOMETRY
An unfathomably long laser beam led cautious guests through the pitch black to the Givenchy show inside Paris' storied Palace of Justice.
The historic stone building that housed Marie Antoinette in the last days before her execution still possesses an eerie quality that designer Clare Waight Keller nicely exploited in her nighttime show.
Thumping and gritty bass music creative a hard atmosphere, alongside the raw warehouse curtain decor.
The clothes themselves weren't quite so eerie.
There was a slight hardness to the graphic quality of some geometric bodice straps, or in the interlocking V-motif on a high collared, ankle-length gown in black.
But there was much more softness among the 59 men's and women's styles, owing to the frequent fluttering of full silk skirts, sometimes in optical print, and the soft colors.
It's hard to be gritty, after all, if you're wearing a spring-like palette of light sky blue, bluebonnet, pigment and cadmium green, and corn yellow, (balanced with off-white and on-trend black).
Some of the simpler looks — such as a series of one shoulder gowns — didn't feel like the talented British designer, alas, was pushing the envelope much this season.
But the styles ended on a high note when Waight Keller got her disco on and served up a silver, Art-Deco style column dress with armor-like shoulders.
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COMMOTION AT VALENTINO
When fashion insiders tried to leave the Valentino venue at the Army Museum inside Les Invalides around 6:00 p.m., a commotion beyond the metal barriers on the street forced the security to close off the exit.
As the crowd of fashion guests swelled into the hundreds, one French editor muttering she was too important to wait pushed past angrily.
After vocal protests over being squashed, an employee of KCD PR agency explained that the police had given them instructions to stop the guests exiting the show, after someone in a vehicle had tried unsuccessfully to swerve into people on the street.
Two police officers on the scene who wouldn't give their names said an unknown person in a vehicle had tried "to ram" police officers outside the Valentino show, but didn't hurt anyone and was detained.
The fashion crowd was directed to exit via the south entrance, past the big Celine show venue with the rival house's name in huge lettering — in what was unfortunately bad optics.
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ANDREAS KRONTHALER HITS THE STREET
Celebrity drag queen Violet Chachki and Milk from "Ru Paul's Drag Race" held court in a warehouse at Andreas Kronthaler x Vivienne Westwood's show.
The warehouse was an appropriate setting for an eclectic assortment of styles that riffed on the "street" in streetwear. The models, both men and women, rode skateboards and scooters.
Prints sometimes resembled graffiti, and there were sneakers, sweatpants and even the odd dog collar.
Andreas Kronthaler, 52, who took over the creative reins from his 77-year-old wife and design partner some years ago, mixed the draped gowns that are a Westwood signature with thigh-length menswear shirts, prints and text.
One of the best looks was a giant pile of fabric bags that were wrapped around the torso of a female model, perhaps a statement about the hectic pace of daily life.