It began Friday evening with the long, multiplying lines outside the gates of Churchill Downs, the all-night encampments in motor homes, tents and sleeping bags which turned into mobile block parties when stereos started blaring and campfires were lit in old barrels.

An enterprising clerk from Ed's Beer Dispensary, near the track, offered curbside service for early arrivals to the Kentucky Derby, delivering six-packs to car windows. A local eatery, The Fat Man's Barbecue, set up shop in a vacant throughout the night and early morning filing the chilly air with irresistible aromas.

Derby Day actually had dawned before midnight. The annual celebration of handsome thoroughbreds and mint juleps, on the first Saturday in May, when Louisville becomes everybody's "Old Kentucky Home," had began.

Financier Louis E. Wolfson's Affirmed, with 18-year-old Steven Cauthen up for his first Derby ride, won the 104th "Run for the Roses" yesterday, holding off his arch-rival of a year ago, Calumet Farm's Alydar, by a length and a half of a stirring stretch run. Affirmed's time was 2:01 1/5 for the 1 1/4-mile gallop on a fast, sunbaked tracked.

Affirmed, trained by Laz Barrera, capture $18 6,900 for his Harbor View Farm, plus the champion's tradiitonal blanket of roses. He paid $5.60, $2.80 and $2.60. Alydar, trained by John Veitch and ridden by Jorge Velasquez, paid $2.60 to place and $2.40 to show, while Hickory Tree Stable's Believe It, trained by Woody Stephens and with Eddie Maple in the saddle, was third, another 1 1/4 lengths back, and paid $2.80 to show.

A crow of more than 130,000 wagered a record $4,425,828 on the Derby.

Rain had been predicted to dampen Derby Day for the first time in more than a decade, but it didn't dare. The day was idyllic, and one of the great celebrations of sport and Americana blossomed, as it usually does with the tulips and azaleas on Churchill Downs' scenic 160 acres.

The thrilling two-minute run by the best 3-year-old colts in the land is the premier event in American racing, but also a spectacle, a mellow Mardis Gras, that transcends those of the horsy persuasion. For many, the race is as much an excuse for lively partying as a focal point of it.

"Ah'd say this is a great cross-section of America," drawled a bearded, gold-toothed good ole boy profoundly, looking out at the mass of humanity on the infield today. "And for every colt runnin' around that ole track, there's a bevy of fillies prancin' around in here. Ah'm here to look at the sem-eye nekked women."

Churchill Downs with its stately antebellum architecture and majestic twin spires, is a shimmering vision rising from the bluegrass. The crowd it manages to pull together this day is no less a sight, even through bourbon-blurred vision.

The clubhouse, box seats, grandstand and standing room area (no view of the track) in the rose garden have their fashion shows. All of Louisville society parades its finery. Politicians from Kentucky Gov. Julian Carroll on down, and such venerable figures as Col. Harland Sanders - resplendent in white linen suit and spotlessly licked fingers - are here to see and be seen.

But out beyond the grandstand, colorfully packed to its 43,000-seat capacity - past the reflecting pools, the horseshoe-shaped winner's circle of red blooms, the flower bed ablaze with violets, geraniums, marigolds and these magnificent tulips - is the infield.

This is where another rite of the season occurs. Most of the 80,000 people here cannot even see the track, but by the eighth race - the Derby - they are so blissfully in their cups they don't care.

This is where bare-chested young men invite nearly as scantily clad young women to tiptoe through the juleps. And presumably - in these days of liberation even in the Bourbon Belt-vice versa.

It is where the college-age crowd spreads out blankets and settles in for a day of serious sun-worshipping, drinking, Frisbee tossing, Wiffleball palying, munching from overflowing ice chests and baskets, and occasional journeys to the mutuel windows - many of which sell only tickets on the Derby.

Several years ago, the nature of the infield scene changed from family picnic to semi-rowdy fraternity bash, complete R-rated sunbathing, the odd X-rated encounter session, and the fragrance of dried grass, rolled and burning, blending with that of the natural variety native of the region.

It has swung back somewhat, but is still dominated by the youth culture, combining equal parts Woodstock, homecoming weekend, and springtime frolic in the park.

A few Derby Day glimpses from the infield:

Nine uniformed Louisville policemen are huddled urgently. Have they arrested someone? Are they administering first-aid to a fallen reveler? No, they're just laid-back cops, all trying to get a simultaneous look at single Daily Racing Form.

A crowd of several hundred people have formed in a circle and are cheering madly. Have they picked a longshot in the last race? No, this is the latest fraternity home competition, apparently inspired by Charlie's Angels: nominated dates in form-fitting T-shirts are having "a jiggling contest," complete with male cheer-leaders.

The tall red-headed man, shirtless and wearing green track pants, looks familiar even behind his shades. Why is he muching strawberries and stalks of celery, slugging from a bottle of apple juice, while everyone else gulps fried chicken, country ham, thoroughbred pie (walnut filling with melted chocolate chips swirled throughout), and other caloric goodies, washing them down with beer or whiskey. He's Boston Celtic center and sometimes Kentucky farmer Dave Cowens.

The hirsute lad is singing "I've Got Pabst Blue Ribbon on My Mind," and one wonders why the hero of his T-shirt is not here. Who might that be? "Billy Carter," it is emblazoned on his chest, "Preaches the Power of Positive Drinking."

Concessionaire Harry M. Stevens has 1,700 vendors working the Derby, 250 of them selling mint juleps. Why do people pay $2.50 for these drinks, described by a Louisville columnist as "a sprig of Salyersville spinach dipped in bourbon and sugar water?"

"I just want to try one, for tradition's sake," says a lovely young damsel in halter top and cut-off jeans. "Besides, you get to keep the glass." As well as the mint.

The tall man in denim overalls, toting a massive bedroll, looks forlorn. Has his damsel deserted him? No, he explains, he's frowning because guards found and confiscated the two half-gallons of sour mash he had hidden in the bulky sleeping bag. "Now I'm going to have to drink beer. At a dollar a cup," he says, "a fella could go broke before the Derby."

A few years back, Churchill Downs authorities barred bringing alcoholic beverages into the track. Men are posted at every entrance to search thousands of bags, backpacks and coolers. Most of the time they do a half-hearted job, and Prohibition does not work very well. Occasionally a boot-legger gets nabbed with the goods in his bedroll; his penance: the exhorbitant prices at the many green-and-red-striped concession tents.

On the whole, though, this is a hassle-free gathering. "You just can't relate this to people, what you see here, a middle-aged man in bright green, double-knit leisure suit said to his wife, surveying the tents, the home-made fraternity flags and banners, the youngsters cavorting merrily. "Mildred, we were born 40 years too soon."

The sky was still thinly overcast and foreboding at 7 a.m. yesterday, but the sun broke through and burned away the gray 45 minutes later. Just in time. At the stroke of 8, the party officially began.

That is when the ropes to the parking lots were lowered, the gates opened and the early-arriving general admissions ($15 a head) scrambled to the unreserved lawn terrace and the infield to stake out thier homesteads along the rail.

Many, including those who had been on all-night vigil outside, wore bulky sweaters, coats, rain gear, but by 9 a.m. the sun was shining gloriously and the Great Peel began.

Four strapping students from Bowling Green came through the tunnel to the infield, which was then still sparsely populated, and breakfasted on Kentucky Fried Chicken. "It's Der-r-r-bee Day," one of them shouted, through hands cupped to magnify his voice. His words rang through the area as stirringly as the bugle that calls horses to the post.

"Aw-ri-i-i-ght," he screamed.

All this brought back memories of a college student from Boston who, on a whim about 10 years ago, decided to make his first trip to the Kentucky Derby. He had spring fever, and the infield beckoned him. He also had met an attractive belle from Louisville, and his imagination was breathing heavily with thoughts of splendor in the blue grass.

The Derby lived up to his grandest expectations for a rapturous day of Southern fried fun in the sun, even if the would-be object of his affections did not. When he got to Louisville, on a stand-by-red-eye flight, he bunked in at a friend's house and promptly called his sweet Mary Lou.

"Are you kidding?" she said, spurning his suggestion that they be a twosome on Derby Day. "This is the biggest social weekend of the year here, the only one. Every girl I know has had a date lined up for weeks.

"But why don't you tag along with our group anyway," she added encouragingly, introducing the innocent to local mores. "Maybe one of the guys will pass out."

Alas, at her front door he found only a note: "Sorry, had to go ahead early. See you on the infield."