Hayek said she was inspired by the female empowerment of the clothes at the storied house whose legacy is of being the first to dress women in men's suits.
Meanwhile, rapper Kanye West and former-Beatle Paul McCartney arrived at the Stella McCartney show buddy-buddy, though their musical styles couldn't be further apart.
Here are the highlights of Monday's frenetic fall-winter 2015 ready-to-wear collections, including show reports for Hermes, Giambattista Valli and Saint Laurent.
SAINT LAURENT GOES SEXY, 80S GLAMOUR
Fashion's wild child Hedi Slimane upped the sexiness in his latest offering for Saint Laurent, with thick eye make-up, whitened faces, torn tights and a flirtation with the styles of the '80s' New Romantics.
The stomping models were still here in this latest chapter of the controversial French-Tunisian's evolution at the house, as were the glittering Glam Rock fleeces, black boots, black leather minis, and the figure-hugging leopard print cocktail dresses.
But it felt like this Saint Laurent girl had grown up a little bit, moved on the early Eighties and discovered sex.
A tight black leather zip-up minidress was undone nearly to the crotch, while one truncated bridal dress was twinned with a tuxedo and looked like the heady bride had ditched her man at the altar and gone to the disco.
There were also strokes of glamour — like a black one-shoulder split minidress that sported a huge white bow.
The show ended on a high — featuring one look with tulle netting coming up from the bust like a visor, and another, a shimmering black microdress with fabulous diagonal insets of polka dots and geometric stripes that put the Saint Laurent chronometer firmly to 1982.
These final looks were some of the most creative Slimane has produced since he landed at the house in 2012.
SAINT LAURENT SISTERS HAYAK AND CHASTAIN
Fashion royalty Hayek, wife of Kering luxury group CEO Francois Henri Pinault, sat pride of place at Saint Laurent, and chatted enthusiastically with Chastain after the fall-winter show.
"We talked about female empowerment... I'm much older than Jessica, but it reminds me of the '80s: a little punk, but chic. A different take on it: very strong, very sexy, very feminine ... I loved every single piece. Well, (apart from) the one with the boob out, (that) I don't know that I can ever wear. Yes, Jessica can," Hayek suggested.
Chastain was a little less enthused with the enforced nudity.
"Well, I thought I could wear that with a bra," replied Chastain, and laughed it off, though the Oscar-nominated actress is no stranger to topless roles, having done so in three films, including 2013's "Salome."
JESSICA CHASTAIN IS A WORKAHOLIC
Jessica Chastain has signed up to a highly anticipated new project: to play Marilyn Monroe in a proposed 2016 film "Blond."
"I'm not ready to talk about it... But I'm very excited to be working with (the director) Andrew Dominic," she said.
Chastain was, however, ready to talk about her admiration for Oscar-winning director Guillermo del Toro, who's directed her in the supernatural period film "Crimson Peak," that comes out in October.
"He's a dream come true. I've worked with him twice — he produced a film I did called Mama. And "Crimson Peak" is the first one that he's directed that I've done, which was really cool," she said.
Chastain, has another film — "Martian" — coming out November and is soon to start filming on "The Huntsman."
"I'm a workaholic," she confessed. "I love it and when I don't work I get to see amazing shows."
Woody Harrelson joined Paul McCartney and West in the front row.
But the strong celebrity presence wasn't needed to attract attention to the 43-year-old's designs.
It was a stellar collection.
Hitting the on-trend spot, Stella McCartney sensually explored styles of undressing for fall-winter 2015, with a hint of the corset.
Gone were many of the sporty shapes that are now associated with her esthetic — and in their place were silhouettes, cinched chicly with a belt at the waist or bustier-shaped. Elsewhere, there were voluminous "comfort" shapes on beautiful loose knit sweater-dresses in earth tones, which would fall nonchalantly off one arm — as if in a state of undress.
Then, in a fabulous series of yeti-style "Fur Free Fur" looks — shimmying past stylishly — McCartney evoked the spirit of her late animal rights activist mother, Linda. They were so well crafted, that even the most knowledgeable fashion editor might not have been able to spot the difference between this fake fur and the real deal — had they not known who had designed them.
They were twinned with great '70s kick flare pants — which together with flashes of eclectic metallic panels — gave the collection a real disco lift.
GIAMBATTISTA VALLI'S INTENTIONAL CONTRASTS
Giambattista Valli combined the sweet scent of the '60s with a play on wildly contrasting patterns for a vibrant and highly original ready-to-wear display.
Starting off with A-line mini-dresses — one with bell (bishop's) sleeves and blocked monochrome, and another with black knee-high lace up boots — the collection blossomed into floral motifs, retro zigzag and check prints on pants and the odd peplum.
Clothes were light, airy with flashes of vivid colour and prints.
But the creative fun really went full throttle on looks that combined contrasting patterns. A monochrome check fitted sleeveless top and pants were worn with an undergarment that peeped out like a peplum in a vivid floral print in black, pink and yellow.
What made the ensemble work was that the diagonal movement in the check was also reflected in a subtle diagonal movement in the floral print. Detail like this is the sign of a true fashion artist, as Valli is.
HERMES' DESIGNER UNVEILS DEBUT COLLECTION
For her debut collection at powerhouse Hermes, new designer Nadege Vanhee-Cybulski pared down the house esthetic, to subtle, yet powerful, results.
Narrowing down the palette to navy, cobalt blue, ivory, carmine red and flashes of sunshine yellow, the 36-year-old French designer used simple minimalist shapes that drew from Oriental styles to bold effect.
The collection was all about gentle contradictions. The austerity of shiny navy leather coats was broken up with a disc panel, or a large soft leather collar.
Elsewhere, a Western-looking sweater sported a Siamese-style split skirt with colour-rich panels, or a shirt in carmine red — associated with the Far East — came on a button-up shirt and was twinned with high sartorial pants. It was a nice mix.
There were also cavalry or equestrian references fused with this creative musing, like a quilted leather coat with large saddle pockets that cleverly resembled a sort of peplum.
Other looks were simply beautiful like an ivory silk split dress with a band on the midriff, which gently swayed as the model walked.
It was a promising start.
Thomas Adamson can be followed at http://Twitter.com/ThomasAdamsonAP