It's . . . coming this way. Oh god. It's hideos--all sweaty and amorphous and . . . look! It must've eaten all the cars on M Street, because they're all gone -- and now it's lurching right for us on 25,000 pairs of legs! Look out! AAAAAIIIIEEEGGHHH!!

Just when you thought it was safe to go back to the Bayou. It's "Halloween in Washington: Invasion of the Alter Ego Snatchers." For one brief, grisly moment every year, they abandon their briefcases, books and blind ambition to become . . . The Thing That Just Kinda Had Fun.

Years ago, Halloween in Washington was a big night for the campus-bound, especially in the lean, loud streets of Georgetown. During the last decade, however, everybody's gotten into the act -- and it is an act -- as this traditional kids' holiday has grown up, and grown to be regarded as "Washington's Mardi Gras," a time when otherwise normal, tax-paying and even tax-collecting adults take to the streets, the bars and to other people's clothing. And go slightly loony.

This year, moreover, Halloween's on a Monday -- and thus the night becomes four nights, starting Friday, of masquerade madness. Maybe you saw "PM Magazine." Well, then you know.

As a public service, however, we'll discuss briefly how a normally sleepy Capital of the Free World becomes a city of cruising space creatures, Marx Brothers and walking trashcans for the sake of fun, fantasy fulfillment and the casting-off of anxiety. We'll also provide a few tips on how to get through it -- and where -- without hurting yourself.

First, of course, the obligatory psuedo-psychology. PSUEDO-PSYCHOLOGY SECTION

(Skip this part if you're under 18; this is grownup talk. At least it's supposed to be.)

"When you get dressed up on Halloween," says Tom Wallich, a partner at Georgetown's Foundry, and a pardner this weekend (in his cowboy suit, he reckons), "you don't have to answer for the things you did last night. Because it wasn't you."

"The thing about Halloween in Washington," says Mike O'Harro, owner of Champions, former owner of Tramps and a fully accredited professor of Georgetown nightlife, "is that this is a very conservative town. People here really need an excuse to party. And Halloween is perfect -- you put on a costume, you come downtown and you can be anyone you want to be. Live out the fantasies.

"People act different," he says. "I've seen women, high- level government bureaucrats, dressed as ladies of the evening. I've seen a lot of really quiet, Casper Milquetoast types dressed as football players, hockey players, Darth Vader.

"Washington is a city of suppressed desire," says O'Harro. "It comes out on Halloween."

It also comes out most clangorously in Georgetown. Why? As the portly, 50-ish woman waiting for a bus at Wisconsin and M said the other afternoon, looking reproachful: "There's so much ugliness here in Georgetown after dark."

This weekend in Georgetown, there'll be a lot more ugliness -- most of it made of latex or greasepaint, but some of it more than skin deep. It'll help that there also will be a lot fewer Metrobuses on M Street. Why?

Roadblocks. Both Saturday and Sunday nights. STREETS FOR PEOPLE (ALIENS,ELVES, ETC.)

People get to feelin' their oats in Georgetown on Halloween," says Metropolitan Police Capt. Rodney Murray. "And it keeps getting more and more popular, with people coming in from as far away as New York -- especially when it's a weekend type of thing.

"We're just going to be a little firmer this year, up front," says Murray, who favors prevention over cure.

In the past, Halloween's spookiest moments in Georgetown, and to a lesser extent downtown in the 19th Street area, were generally brought on by the timeworn friction between the ghosts in their cars and the goblins on foot. When as many as 25,000 costumed revelers descend upon Georgetown -- which has roughly 17 square feet of sidewalk and approximately six parking spaces -- there's bound to be trouble

In the past, police have closed off M Street or Wisconsin Avenue to automobile traffic when things got really bad -- when pedestrians began kicking at doors, or, as happened recently, when a particularly rowdy mob came dangerously close to rocking a Metrobus onto its side.

"After the group dressed up as doctors started performing surgery on the hood of a car stuck in traffic," says Tom Keeley, a Georgetown veteran who now owns Cagney's at Dupont Circle, "that was it. It just snowballed."

That was the past. This year, Murray says, M Street will be closed to automobile traffic at 8:30 p.m. between Key Bridge and 29th Street both Saturday and Sunday (which police expect to be the most crowded nights, though extra manpower is planned Friday and Monday nights as well). Wisconsin Avenue will be similarly blocked off, from K Street up to P Street. Parking will not be allowed anywhere along the blocked-off areas. Traffic will be rerouted to the Whitehurst Freeway and Q Street.

The streets will belong to the people in costume, in other words. Plus about 500 others in police uniforms.

Real police uniforms.

"Wish us luck," Murray says. A BRIEF HALLOWEEN ANECDOTE

A group of men dressed as Perdue chickens picks its way through stalled traffic on M Street.

Someone yells, "Why did the chickens cross the road?"

One chicken turns around: "To get to another bar."

Talk about funny. HALLOWEEN SAFETY TIPS FOR GROWNUPS

The kids hear all about reflective clothing and tampered- with treats. Adults could use some advice, too, particularly when the tricks are downtown and the treats are on the rocks, with a twist.

There are many ways to ensure oneself a safe and sane Halloween. Some of them follow. 1. Stay home. (Turn off all the lights. Otherwise little people arrive, wanting candy.) 2. Stay on the road. If you wear an over-the-head mask or something else that restricts your vision, do not try to drive with it on, no matter how incredibly hilarious this would be. "We hope grownups would have enough sense," says Susan Chapin of Stein's Theatrical & Dance Supply Centre in Arlington, a major local masquerade dealer, "but some are worse than the kids." 3. Stay offthe road. If you're going to Georgetown in a car this Saturday night, leave now. Traffic's bad enough on a Tuesday night in August at Wisconsin and M, so consider taking a bus (call 637-2437) or walking -- from upper Wisconsin Avenue, K Street under the Whitehurst, Georgetown University, M Street east of Rock Creek Park, the Foggy Bottom Metro stop, even Rosslyn. And by the way, can you walk in your costume? Can you breathe in it? Aha. 4. Know your costume. You can fool your friends, but you can't fool your body. In other words, before you staple all that fur to that hoop skirt, or strap on those orange crates, or whatever, think for a moment of the dimensions of the rest room at the American Cafe. And, perhaps you'll want to dance. Is there enough play in your silver garbage bag? Ventilation? Some people do plan ahead, and Greg Doherty, who runs Nathans, remembers one well -- a man dressed as a robot, with a companion dressed as a princess who translated his drink order. When the drink arrived, Doherty says, the robot placed his "hand" in the drink, and then drank it. Surgical tubing, probably. 5. Know your history. Halloween is traditionally wildest in Georgetown, particularly after about 9, but Georgetown's also the most crowded, saloonwise. Arrive early if you want a seat. If you want to cruise, to show the world your Halloween getup -- and most do in Georgetown -- you may make the 11 o'clock news before you make it to the bar at Paul Mall. Less crowded but fairly street-happy are the 19th Street, Dupont Circle and Adams Morgan areas. Less crowded still is Old Town Alexandria. HAUNTS

Like the previous list, the following sampling of places prone to celebrate Halloween is highly arbitrary and so incomplete, it's scary. Just about everybody has a Halloween party. If you simply can't locate one, visit a hospital and give blood. SCARY FILMS -- Try "The Howling," free Friday night at the Hirshhorn (357-1300), or two James Whale classics, "Kiss Before the Mirror" and "The Old Dark House," free at 7 p.m. Monday at the Library of Congress (287-5000). SCARY FILM CONCEPT -- The 301 Drive-In in Waldorf repeats last year's dusk-to-dawn "Horror Movie Marathon" Friday night and Saturday. For costume contest and consecutive showings of "Invasion of the Body Snatchers" (the newest one), "The Hunger," "Still of the Night," "The Grim Reaper" and "He Knows You're Alone," admission's $4.50 -- or $3.50 before 7:30. Free coffee, donuts for survivors. Call 843-6280. GAITHERSBURG HOLIDAY INN -- Annual masquerade ball sponsored by Gaithersburg's parks and recreation department is Friday night at 9. Music by Nightwalkers, cash bar, tickets at door $6.50. Call 948-3220. LAST-MINUTE COSTUMES -- Hartke Theater, Harewood Road NE, 635-5365. Catholic University's theater holds its annual fund-raising costume sale Saturday, 10 to 2. CAPITAL HILTON -- 16th and K streets NW. Saturday night is the Washington Association of Black Journalists' second annual masquerade ball. Tickets $35 per pair at the door, to benefit journalism scholarships at Howard University and UDC. FISH MARKET -- 105 King Street, Alexandria, 836-5676. Annual Pumpkin Festival costume contest registration is 8 to 10 p.m. Saturday, allowing visits to Old Town's relatively quiet streets and taverns before returning for the judging "parade" on the sidewalk out front at 10:30. (Same party takes place indoors at the Fish Market in Georgetown.) RAMADA RENAISSANCE -- 1143 New Hampshire Avenue NW. Saturday night at 8, this Ramada and the Easter Seal Society for Disabled Children and Adults are throwing a "Fantastic Fifties Futuristic Halloween Masquerade Party." Whatever it is, tickets are $10, music is from the '50s and '60s, costume prizes will be awarded by WMAL's Bill Trumbull and Chris Core, and proceeds benefit the society. Call 232-2342. WAX MUSEUM -- Fourth and E streets SW, 554-2600. Grand prize in Saturday night's costume party is two one-year passes to the Wax, where the new video system will mix rock video with Halloween fare, including homemade sci-fi spoofs by the Langley Punks. Music by the Bobcats. Admission's $3.50. J. PAUL'S -- 3218 M Street NW, 333-3450. Though walls and waiters will be decorated and the floor crowded all four nights, the official costume party here is Sunday night, with prizes of free dinners and Dom Perignon. Manager Barry Silverman's suggestion for those not in costume: "Pretend you work for the FBI." RUMORS -- 1900 M Street NW, 466-7378. Sunday night is Rumors' "Hollywood Halloween" party, for masqueraders with a movie-star bent. (Saturday night's "Mardi Gras" night at Rumors' other place at 1716 H Street NW).