The massive crowds outside the huge temporary tent bearing the fall collections of Jean Charles de Castelbajac swelled toward the entrance like the rush-hour mob for a New York express subway.

"You know," said one New York store president, "Each year it gets harder and harder. And each year it becomes less and less worthwhile."

So began Important Saturday here-the day the French read-to-wear shows for fall 1979 opened. Several showings took place earlier in the week, but Saturday has come to belong to the avant-garde Paris designers such as Claude Montana, Thierry Mugler, Jean Claude de Luca, Issey Miyake and Castelbajac. So the biggest crowd of buyers, press, pals of the designers and endless hangers-on, come to make the scene.

In the past, the showings have taken place in halls scattered in all parts of Paris. But this time, with a strong push from Pierre Berge, Yves Saint Laurent's business partner and head of the Chambre Syndicale (the organizing body for the top Paris creators), shows are being held in four specially built tents placed plunk in the middle of the construction at Les Halles, the old market section and now the "SoHo" area of Paris.

A good idea, right?Theoretically, no hassle getting to the shows, good restaurants and shops nearby, space for all. The Beaubourg Museum with exhibits by Kandinsky and Magritte not far away. Tea served by sari-lad ladies and coat-and-tied ushers to show you to your seat. The tents themselves light and airy, with many good seats.

The problems is that there is no small ego trip in this business. So while the tents hold from 700 to 1,500 seats, designers send out tickets to all their pals, who all come and bring their friends, and all cheer madly for the theatrics onstage.

Even Kenzo, who last year abandoned the big show and presented his clothes to small groups with tea and television in his Place Victoires studio, has gone back to the grand show concept. "A desinger just seems to need the applause," says Gilles Raysse, Kenzo's business partner.

So when 500 buyers and 900 press from around the world,-that was the count according to the Chambre Syndicate-show up for their legitimate assigned seats, of course the crowd is almost impenetrable.

If there are fewer Amerians present this round, it doesn't show on Dubois' list. And the message buyers have gotten from the designers in Milan and London last week and in Paris during the opening shows, is very clear:

The silhouette generally is broad-shouldered, narrow and short.

Colors are very strong, often mixed with neutrals. Bi-color outfits favored in Milan give way to tri-color combinations in Paris. Inter pastels and classic tartans are beginning to show up.

Fitted suits (pantsuits, short jackets, one-button draped jackets, reed slim skirts, peplums) underscore that the carefully planned costume has replaced thrown-together separates. The evening suit in fancy fabrics is clearly being revived.

Two important coat shapes-the fitted redingote and the 7/8 coat which seems to best complement the broad-shouldered, narrow proportion.

Shoes have stilletto heels or cone-shaped heels in spite of how difficult they are to walk on. Wooly tights and dark hose are paired with bright colored pumps.

Things spiral around the body in stripes, even crocodile patterns to emphasize the skinnier shapes. Earrings, ring bracelets, spiral and wrap. Skirts spiral around, giving uneven hems a new emphasis.

Everything is gussied up with hat, gloves, jewelry carefully picked to match. Nothing casual, everything planned to the last detail.

Buyers are meeting upscaled prices-increase of 10 to 20 percent over a year ago - with orders of less units that total about the same money amount spent for last fall.

But the gnawing question many buyers are asking is what do huge shoulders, theatrical-style clothes and outer-space outfits, vampy suits and comic strip clothes have to do with today's woman?

"There is so much creative energy here," says Neal Fox of I. Magnin, "but I keep asking - where can my customers wear it? And sometimes I have to answer - no where at all," Fox said after the Montana show. He was pleased, he said, with the Italian shows of Missoni, Armani, Basile, Fendi, and Krizia.

"If you had tried to sell big shoulders in a store this winter, you wouldn't touch them for spring," insists Val Cook, of Saks-Jandel, about the padded-shoulder styles, some of which looked like the model had forgotten to remove the hanger before putting on the dress.

Marvin Taub of Bloomingdale's kicked off the party season in Paris Saturday night to celebrate the store's tenth year of doing business in a sizeable way in France. According to Traub, Bloomingdale's chairman, Bloomingdale's expects to do between 35 and 40 million dollars worth of business with France this year, with clothing the major share.

Claude Montana, who was the first with the biggest shoulders in Paris, proved he was a true believer with his own garb at the Bloomingdale's party. He was wearing his usual jeans but had padded the shoulders and sleeves of his American windbreaker to gain a Popeye look. CAPTION: Pictures 1 and 2, Big-shouldered coats for '79 by Jean Claude de Luca, left, and Claude Montana, UPI photos.