The designer died at his Paris home following a long illness, Pierre Berge, his longtime friend and business partner, told the Associated Press.
"The house is very calm today to pay homage," she added.
She described Saint Laurent as "the most important designer of the 20th century. He invented so many codes of modern elegance. He watched so much change in society. He's more than our national treasure."
She added: "So many of the codes of our brand that he invented, like the tuxedo, are still alive."
Also Monday, a statement by Pierre Bergé from the Yves Saint Luarent foundation in Paris said the funeral will be held Thursday afternoon at the Saint-Roch church in Paris.
President Nicolas Sarkozy is expected to attend the ceremony, the statement said.
Saint Laurent will then "be incinerated and his ashes will be kept in a burial in the Gardens of Majorelle" (in Marrakech). Pierre Bergé and Yves Saint Laurent had bought the Gardens of Majorelle in 1980.
From the start of his career at 21, when he replaced his mentor, Christian Dior, as chief designer of the couture house of Dior in Paris, Saint Laurent crafted a modern look for women that set a new standard.
He was the first to make pants and pantsuits the basic pieces of a woman's wardrobe, doing it in a way that conveyed femininity, self-confidence and style. In contrast for evening, he styled sheer blouses, flounced skirts and a slinky tuxedo worn over bare flesh that he famously named "le smoking."
"The word 'seduction' has replaced the word 'elegance' in fashion," a French television commentator announced in 1967, about Saint Laurent's effect on the industry.
The French designer's gift for redefining French couture was apparent in a single dress he showed in his first collection for Dior in 1957. A "trapeze" style, it fell in loose folds from the yoke to the hem with no padding, no whalebone construction, no corseting. The easy shape and loose fit were younger, freer than anyone thought of as haute couture, a world dominated by designers in their 70s.
For the first decade of his career in fashion, Saint Laurent continued to startle audiences with his innovations -- a Navy peacoat, a "beatnik" motorcycle jacket, a dress that looked like a Piet Mondrian painting.
His designs were "the antithesis of the haute-couture school, with its premise of buttressing and correcting the woman's silhouette," wrote Alicia Drake in "The Beautiful Fall," her book about French fashion during Saint Laurent's rise to fame.
At the top of his form in the 1970s, Saint Laurent had amassed a fashion empire that included a couture and ready-to-wear division, best-selling perfumes starting with Rive Gauche, and licensing agreements that put his name on sunglasses, hosiery and more than 100 other products. His estimated annual take-home pay was about $4 million.
As often as he was declared a genius, however, Saint Laurent was described as a tormented man who struggled with clinical depression, alcoholism and drug addiction. At low points, starting in the 1970s, he was hospitalized, discharged only long enough to oversee his latest fashion show and then whisked back to confinement. His emotional problems worsened later on.
He portrayed himself as a suffering artist while his business associate, Berge, tried to make light of it. Saint Laurent "was born with a nervous breakdown," Berge said.
Shy and illusive, 6 feet 2 and exceedingly thin, Saint Laurent looked like a gangly schoolboy through the first decade of his career. He had moved to Paris in the early 1950s to study fashion at the Ecole de la Chambre Syndicale, the school overseen by French designers. He captured early attention by winning a design competition sponsored by the International Wool Secretariat in 1954, with his sketch of a dress with one bared shoulder.