TALK ABOUT street theater! They're fighting World War II down at the Apex Building on lower Pennsylvania these nights: gunshots, floodlights, Nazis with police dogs, lots of screaming and singing and dying and sirens and a horse.

Three hundred people follow the action of the Zagreb Theater's "Liberation of Skopje," herded from park to bleachers, frantically reading the translation as they go because it's in Serbo-Croatian.

Just the same, it gets to you. At the end, excited audiences usually give a standing ovation and try to join in the Yugoslavian singing.

But you don't need to spend $10 to see street theater. It's everywhere. Especially in the summer. Especially at lunch hour.

Neat people in seersucker are swarming around International Square at 19th and K. Something going on: balloons, banners, amplified music. Bob Devlin, a one-man band, is pounding away on his guitar, blowing into his harmonica and stamping the drum and cymbal pedals.

"Here's 'Midnight Special,' " he shouts into the mike. He wears his usual leather cap and vest and rolled-up sleeves. His legs go like pistons, doing a square dance sitting down. "This is for muscular dystrophy, folks. Your money goes to the M-D Association." A yellow pail in front of him says "Hello, my name is bucket. I eat money."

People are selling T-shirts nearby. Devlin himself, who has played the White House and Kennedy Center, is hawking his record album, "Bob Devlin Live at 18th and M."

A bearded Sikh in a purple turban stalks past the scene, turns, gawks, grins.

Gallery Place: the annual fair of the Department of Environmental Services. Lining the mall: garbage trucks, sweeper trucks, leaf-gathering trucks, cable trucks, drain-cleaning trucks--and a Frito-Lay truck.

At exactly noon, a rock band rips the sky apart. People roam among the vendors of jewelry, ice cream, incense and sneakers, gape at displays about air pollution, water pollution, noise pollution (rock bands: 120 decibels; rustling leaves: 10), and a crowd sits on the steps of the International Trade Commission building. Nearly everyone is eating something.

A skinny bald guy gets out in front of the band and starts tap dancing, hat in hand. Is he maybe some famous old hoofer? No, he stops after a little while and lopes off gesticulating.

A few blocks away on G Street a freckled kid with a weedy beard rattles coins in a paper cup and timidly sings a hymn, "Teach me to pray . . . things will be better." Nobody stops.

An anti-Khomeini demonstration dominates the K Street side of Farragut Square. People hold up a row of big signs on sheets while a woman's voice drones over a loudspeaker about injustice and murder, women and children. Others sit in a SAVAK electrical torture chair, stand in a little jail with hands gripping the bars, kneel before a firing squad in an execution tableau complete with bloodied victim, while a man in Khomeini mask and robe strolls about furiously.

The lunch eaters pass, trying not to catch the eye of the man handing out leaflets. Across the street, sidewalk traffic is blocked by people stopping to read the signs at a safe distance. Now and then someone accepts a pamphlet or even says a few words to the man.

On the lawn, people sprawl comfortably and spread their sandwich papers before them. Voyeurs patrol the paths watching for women who sit carelessly. A bag man has a whole bench to himself. He is rummaging in his shopping cart full of plastic bags. He moves with great deliberation but little interest, as though he weren't actually looking for anything but simply rummaging for the pleasure of rummaging.

Certain blocks downtown are so clogged with vending stands that they look like a Cairo souk. Some vendors now have outreach programs, helpers who stand far down the street handing out fliers advertising the wonders you are approaching. And sandwich boards seem to be proliferating.

Pedestrians are thus bombarded, not only by the products themselves--from ham-and-cheese croissants (a concoction only an American could imagine, let alone eat) to laser paintings, from hashish pipes to free makeup jobs--but also by the concept of products. Some others are pushing political or spiritual messages.

Eat! Buy! Believe! Will sidewalk hawkers destroy at last the traditional Washington Walking Trance like aerosol sprays disintegrating the Van Allen belt?

At 35th and P, a man stands at an easel painting a Georgetown scene. His audience is a small girl, who says nothing. He paints on in silence. They seem wedded.

Window display of the week: Clement's Caterers and Pastry Shop, 1338 G St., has golf clubs and tennis racket propped up beside a tiered wedding cake, conveying a jaunty, if indefinite, message. The groom on top is in Air Force blues. Another cake has a child's tea set on it, and around it you see model racers, sailboats and Snow White and the Seven Dwarfs. There is also a set of birthday cake candleholders in the shape of pink booties. Amazing.

The bench sitters at Dupont Circle are ignoring the slim young preacher in shirt and tie and rimless glasses who reads from the heavily placemarked Bible held familiarly on one spread hand.

"The Bible says we all have sinned, and sin is disobedience to God," he thunders. One senses that this does not come easily to him. "The Bible declares" -- he riffles pages -- "one day you will have to die because you have sinned."

A man reading a Dick Francis mystery glances up, his concentration broken.

"Jesus Christ died for you and for me," the voice booms. He is beginning to sound hoarse.

A blond girl stares at the preacher as she passes, then continues walking away from him. She is still hearing him, though. She raises both her arms dramatically as if being crucified and laughs to her companion.

It is rush hour, and cars are pouring out of the L Street parking garages. At 18th Street two motorcycles and a police van are pulled up on the sidewalk, and four cops are huddled around a vendor who is telling them he does too have a license.

The traffic light just above their heads is frozen, and traffic quickly backs up on L clear beyond New Hampshire Avenue, blocking that street too. Drivers are honking. Drivers are shouting. Pedestrians pile up on all four corners of the intersection. Nobody dares run the light.

The cops are still talking to the vendor. The light is still frozen. Now pedestrians are stopping to watch, to see how long it will take those guys to notice what's happening.

"They probably aren't traffic cops," someone snarls. "It's not their job."

It's been five minutes, and they're still talking . . .

A well-dressed middle-aged man with a gray mustache and briefcase approaches and says, excuse him, he's so embarrassed, but his wallet has been stolen and he doesn't have a nickel on him and he needs $1.20 to get back to Rockville, and if you give him your address he'll send you a check tomorrow. It's all so ridiculous, he's never had to do anything like this before, et cetera.

Completely convincing. Honest eyes.

The only trouble is, he came up to you a year ago with the same story.

A giant crane hovers by a 10-story building shell. A barechested guy in a hard hat yells down, and the operator swings around in his cab. Slowly, gingerly, he moves the steel arm away from the building. With an excruciating whine the extension retracts. People cover their ears. A man steps out into the crowded street, already reduced to one driving lane by the truck that is the huge crane's base.

He waves his red flag. Traffic waits impatiently. Everyone stops to watch, head tilted back. The crane arm is inching lower. Its engine roars. In the cab, the operator shields his eyes from the evening sun as he guides the big arm past a scaffolding. It takes forever.

Finally he brings it neatly down onto a special stand in the parking lane. The flag man yells, "You got it, you got it!"

And steps up and unhooks from the crane's business end a two-foot hunk of cable.

That's theater.