You hear it first, the roar of it foaming up over the iron grillwork and live oak trees of the French Quarter, a breathy, echoey roar, like a gang war going on in a stairwell, or a World Series crowd trapped in the Baltimore Harbor Tunnel.

Except it's a constant roar, like a storm surf, if you can imagine a storm surf of screaming bodies, bagpipes, stuck car horns, the hackle-raising scuffle of panicked feet, ululations like the last charge of J.E.B. Stuart's cavalry in full rebel yell.

The bleating of those long plastic fanfare trumpets people drive you crazy with at football games, a wino shouting "I am unique!" over and over and the clatter of hundreds of thousands of feet mashing through millions of cans, bottles, laughing gas cartridges, and the plastic and paper "go cups" for booze to go, this last a sound to be reckoned with all by itself--a noise like the universe grinding its teeth or the soundtrack in the science movie when they amplify the chewing of a termite 10,000 times--this is what notifies the visitor nearing Bourbon Street that it's Mardi Gras, even if it's only 10 o'clock on Fat Tuesday morning.

Nothing is real, people say down here. And there's always a parade. Always. It takes only one or two people if they come on strong enough, like the blood-smeared dwarf toddling down St. Ann Street dragging a whip. His name is Wally Sherwood, and he says, "Mardi Gras is when fantasy becomes fact and fact is fantasy." The side he's on is "fact," he says.

Not far behind come seven people dressed up as a streetcar, and nearby is one of the hundreds of evangelists who push through the crowd bearing signs--Jesus loves you--and huge wooden crosses daubed with red paint. Things have fallen apart, a beer-dimmed tide is not only loosed upon the world but is flaunting it, parading it, in front of God and everybody--God to get his chance on Ash Wednesday, the beginning of Lent.

It is Fat Tuesday morning but it doesn't make much difference what time it is. On the last four days, Carnival is in the home stretch, it has momentum, a kind of happy and doomed inevitability that's been building since Twelfth Night, technically; or for 11 days since the first of the 58 parades listed on the official Mardi Gras calendar, or all weekend while the French Quarter swells with a heavy-duty party crowd--dreamers and screamers, midnight cowboys and noonday Draculas from all over the world. For a lot of New Orleans, Mardi Gras may be the assertion of social prerogatives and conspicuous consumption, but the French Quarter is where you end up if you're the kind of person who starts out for parties drunk, if inspiration seized you in a far city and you realized you could be there for the world's biggest free show if you left now and drove 19 hours straight through.

"Ahloveitahloveitahloveitahlove- itahloveit!" a woman is screeching on lower Bourbon Street. She's from Australia. Her name is Sandy. Actually, she's enraged. She's stalking around shouting abuse at a woman up on a balcony.

"Y'all cheeky!" she says, trying on a southern accent. "Go for the real thing!"

The woman on the balcony wears a T-shirt reading "Drop your drawers," but what Sandy is after is what chanting hordes under balconies throughout the quarter are after, which is for the women to hoist their blouses, pull down their tank tops, turn on their love lights, the chant going: "Show-Your-T's! Show-Your-T's!"

How strange. On a street lined with porno arcades, and every conceivable visual combination of naked vertebrate life, people will stand under balconies for hours beseeching women with shouts, Mardi Gras beads and doubloons. The women in their turn may choose to tease for hours, like the one biting her lip and flashing her eyes over the street full of people, which includes Sandy.

So innocent! So passionate! "Look here!" Sandy hollers and hauls down her tank top, and the woman up there smiles, everybody smiles, cheers, waves. Eros, as opposed to Pornos, is rampant.

"S'Mardi Gras, gimme kiss," says George Weatherton, a sales manager with his face blazing with red greasepaint and a 32-ounce cup of beer in his hand. There's a lot of kissing: precise, ceremonial smacks between strangers, everybody knows the rules, you don't just walk up to some strange girl and start gnawing on her face, but there's something about George--he keeps trying to smear everybody else with red greasepaint too--that turns off the object of his affections.

"I hate rejection," he shouts cheerily after her. "For about five minutes."

S'Mardi Gras. Shrove Tuesday. A bacchanalia, a debauch, an orgy. Saturnalia, Lupercalia, the Hindu holi Festival, Germany's Fasching. Writes the worldly James G. Frazer in "The Golden Bough": "We have seen that many peoples have been used to observe an annual period of license, when the customary restraints of law and morality are thrown aside, when the whole population give themselves up to extravagant mirth, and when the darker passions find a vent."

For darker or lighter, a trio of men in lingerie, wigs and work boots sashays through the crowd, gloriously grotesque. "Don't you love Danny's earrings?" one of them says. Danny wears pearl chandelier earrings. "She was visiting in the Chandelier Islands."

"Yes," says Danny, "I picked 'em up on the beach."

Pre-Lenten festivals have been around for a long time, and the French settlers in New Orleans held mass balls throughout the 18th century. But Mardi Gras as we know it didn't get fired up until 1856 when a group of young bloods from Mobile, Ala., met in the drugstore of Dr. J.H. Pope and formed the Mistick Krewe of Comus. They dressed up as the demon actors in Milton's "Paradise Lost" and, with the French population complaining about Anglo-Saxon effrontery, they rolled down Magazine Street on two floats lit by torches carried by black men; the Daily Delta said it was "more startling than pleasing." The next year, Comus had 30 floats. In 1870, they were joined by the Twelfth Night Revelers. In 1872, Russia's Grand Duke Alexis showed up in pursuit, rumor had it, of an actress named Lydia Thompson, and accompanied by Gen. George Armstrong Custer. To entertain the duke, a new Krewe Rex, was formed and they led a bull named Old Jeff through the streets. This was the boeuf gras, the fat beef symbolic of Carnival which is usually said to derive from the Latin for "farewell to meat." Carnival, American Style

It has grown to the Carnival done up American style, done up big. This year there are 49 balls at the Municipal Auditorium alone. Fifty million dollars is spent for Mardi Gras, according to Blaine Kern, who built 250 floats this year, at a cost of up to $50,000 for the Krewe of Bacchus' gigantic Bacchusaurus. Kern minted 25 million--that's million--doubloons for this year. Doubloons are silver dollar-sized aluminum coins that members of the Krewes throw to the crowds from the floats--along with a hailstorm of bead necklaces, as the floats grind along five miles of various parade routes, throughout the city and the suburbs while the crowd shouts "Throw me something, Mister!" (this being the traditional cry) and waves hands in the air. The New Orleans Tourist and Convention Commission estimates that a million people may be on a 15-block stretch of Canal Street alone.

"It's a form of public madness," says Munro Edmonson, professor of cultural anthropology at Tulane University. "It burgeoned after the Civil War as a nativistic movement, glorifying a past that never really existed anyway. It's an explicitly anti-democratic attempt to perpetuate Louisiana aristocracy."

Endymion is one of the newer Krewes, and its parades feature celebrities (blue-blood Comus never reveals the name of its monarch). This year it's Kool and the Gang, the Captain and Tennille, and Doc Severinsen, up there on the floats, heaving junk treasure. Not to mention big Lou Cannizaro, who spends about $7,000 a year on Carnival, over a thousand of it in beads and doubloons he unloads on the crowd. He wouldn't be in one of the more upper-class Krewes such as Rex or Comus. "I'm a happy-go-lucky guy," he'll say. "That's outta my class, you know what I mean."

Nevertheless, the old-line types get up in the reviewing stands in front of the Pickwick and Boston clubs, the men in neckties (white tie for the Comus procession), little girls in white knee socks, all of them waving their hands like the folks down on the street: the kids with the giant radios, a woman holding a baby in one arm while she goes up for beads with the other. Manna! Pennies from heaven! Luck! Grace! Love!

Love, oh loveless love.

Endymion's floats roll past and a woman named Linda has her hands in the air while she explains she's here looking for a husband who left her in Oklahoma after an argument in which he threatened to bash in her grandfather clock and she told him to leave.

"He keeps writing me from New Orleans, he wants me back, and I write him back at General Delivery but he never picks them up. It's been a year and a half. He keeps writing me and saying why don't you write me back? I decided I'm going to live in either L.A. or New Orleans, I can't stand it anymore. If I move to L.A. he'll never find me, so I came here for one last try, I've been driving since 8 o'clock this morning, fog all the way. He doesn't know I want him back. He wrote me from Fort Worth and I wrote him, I said hold on, I'm coming, and when I got there on the bus it turned out he's just taken the bus over to New Orleans, we passed on the highway. I could have anything in the world but if I could see his face in the crowd . . . but he probably isn't in the crowd, he's probably in a bar. One time when I was drinking a glass of wine with another man he went out and smashed in all the windows of my station wagon with a baseball bat. That was a real put-down."

She had her hair done and dyed two days ago. She's looking. She's hoping.

Loveless love: On Mardi Gras day, a 7-year-old girl is crying "I want my mama!" while a Marine Pfc. takes down her name and description at the lost-child compound on Canal Street. Next to her, a 10-year-old girl says "Yes," she just turned around and her mom and dad were swallowed by the crowd, and then she can't say any more for her lower lip trembling. These are children Nos. 31 and 32 to be abandoned this morning. "What we've suspected is that people will lose their kids on purpose," says Police Capt. Kenneth Dupaquier. "We pick up a lost child at 10 a.m. and we don't get any inquiry until 6 or 8 p.m."

Anything goes. The Bourbon Street crowd will part with hardly a murmur for hundreds of evangelists from a group called Christ for Nations who stride through the screaming throng bearing crosses and refusing, for once, to say anything until they break into a minor-key dirge of "Be Strong and Conquer." Very grim, a real medieval plague-procession aspect to it--and these folks are waving their hands in the air too, though slower than the parade crowds, a gesture as if they're wiping a very fragile sky clean. They do not diminish the revelry.

Whatever gods that Mardi Gras petitions have a strangely impassive aspect. The huge fabricated heads on the floats glare with solemn preoccupation. All the riders are masked. The women on the balconies smile small smiles. The crowd beseeches: Throw! Show!

Mardi Gras seems to constantly gain speed and inertia. Music pounds out of bars, strip joints, giant radios and strolling Dixieland bands until the air is gathering in a single giant metallic heartbeat. Over by the fisheries and wildlife building on St. Louis Street the magnolia trees are full of screaming men. Bourbon Street is sticky with beer, it feels like it's pulling up in strings under your shoes, the whole scene thick with the smell of a combination of, say, low tide and a wet dog. The Krewe of Pharts--there are lots of mock Krewes satirizing the established Krewes--wanders around in robes that tout the New Orleans specialty of "Red Beans and Rice."

Up on the balcony of the Royal Sonesta Hotel, Sandy Duncan, the actress, watches the protoplasm writhe beneath her and says, "The crowd's getting a little scary." She is here to reign as queen of the Krewe of Zeus. "One thing I've learned, I think at the end of every curtain call, I'm going to throw doubloons." Just down the balcony from her, a woman waves a pair of giant breasts she made out of stockings and pillow stuffing. The crowd heaves beads at her. Never in New York

"In New York this would turn into World War III," says Patti Fratalia, who works for a tanker brokerage there. Police Officer R. J. McWilliams says yes, "people from up North can be a problem--they don't understand how the police work down here." Which is very laid-back on venial sins, but when a kid disobeys an order to move on, a cop will take after him like a drill instructor, bellowing: "You think you're special, buddy?"

How else do you handle somebody dressed as Santa Claus, or Rubik's Cube? Bruce the shark? Why be subtle at an endless party where seven entrants in the gay fashion show appear as Mommie Dearest, whipping baby dolls with coat hangers? The odd thing is that the outrageous and the ordinary acquire the same values. People will amass outside the window of a Ramada Inn to watch someone play with a souvenir hand puppet.

Over on Dauphine Street, in front of the Whiskey River Bar, a skinny blonde with a collection of homemade tattoos--Little Devil, a pachuco cross, a butterfly on one shoulder blade across from an open sore on the other--lingers half-naked and largely unnoticed accompanied by what somebody calls "the Whiskey River crowd--they know what to do." Which is nothing until some middle-aged type with an Instamatic tries to open fire on her, and a guy in a denim jacket tries to hustle him for a dollar: "We're making drink money with her." Up the street, a totally naked man mounts the side of a moving Vega, much to the surprise of the woman in the passenger seat. The crowd cheers.

It doesn't matter--nothing is real, no offense meant, none taken. Everything is explained, forgiven and forgotten by Mardi Gras.

In back of Jackson Square, by the river, the Christ for Nations people are invoking not Caesar but God, looking skyward as they cluster around a little spastic boy in a wheelchair. "Loose him, Jesus," they say. "Blood of Jesus, loose him, Jesus," and then one of them drifts into speaking in tongues, a sound not unlike the roar of the whole crowd.

The kid twitches, rolls his eyes, grins, slobbers. The healers won't take no for an answer. They haul him out of the wheelchair and hold him while they walk him up and down the steps of the levee, until it becomes apparent the Lord is not going to signify just now.

"We'll let you rest," a woman says to the boy. He flails his arm. A doubloon sails out of his hand and onto the ground. Nobody picks it up. The only sign from the sky is a gigantic flock of starlings screaming as the sun sets behind St. Louis Cathedral.

Mardi Gras ends with a terrible new roar, at midnight. A police bullhorn announces: "Mardi Gras for 1982 is officially over. You must clear the street for the street cleaners." Behind the bullhorn come four cops on horses. The crowd parts. Somebody says: "Why now?" It's midnight, Ash Wednesday and the industrial bellow of truck engines comes up Bourbon Street behind phalanxes of police cars, all with their sirens warping up and up, a machine scream that has people sticking fingers in their ears. This is the voice, as it happens, of reason.

"I hate to see it, it's so morbid," says a nurse named Therese Killeen.

Fat Tuesday thins into the small hours and by morning the natives say, "You'll never know it happened." This is a favorite boast--or are they just placating the rest of an America that can't believe in its Puritan heart of hearts that such license could end in anything but eternal deracination, decay, depravity beyond redemption?

This is what Ash Wednesday is for. Now everything is real, and there are no more parades.