Friday night in the Bronx and a gang of tough-looking kids in red berets and studs and T-shirts are waiting for the No. 4 Woodlawn Avenue train, the "Mugger's Express" that runs through the Bronx subway stations where there are neither lights nor cops.

The kids in the red berets look like they know something about mugging, too.

There's 16-year-old Zem, who used to run with a gang called the Ching-A-Ling Nomads, before, he says, they gave him the boot without the sock. Or Cato, 18, who served time as a juvenile delinquent upstate for ripping people off in the street. Or Rocky, 21, whose former life in petty crime ended when he was found in a Queens nursery school with his hands in the petty cash.

But all that is behind these kids, or they couldn't be in this gang. They've got jobs, or they're in school, that's one of the rules. They play around in uniform, they carry a weapon, they get into trouble on a patrol, they're out.

And when they get on the Mugger's Express, spreading out one to a car, they'll watch it like cops. They'll signal to each other if they spot a troublemaker, and descend on him in a pack. Unarmed, they'll escort him out of a train or make a citizen's arrest.

A regular bunch of tough-guy Good Guys, if you buy what they're selling. They're a kung-fu kicking, mean but clean, down-to-the ground bunch of ghetto Boy Scouts.

The group is the Guardian Angels. They number over 700 in New York City, with groups spreading across the country. Their avowed goal, in the words of their leader, 26-year-old Curtis Sliwa, is "to bring back the values of 40, 50 years ago . . . when the only criminals in the streets were the racketeers -- when you could leave the house with the door open, or fall asleep on the roof. . . ."

They ride the subways or patrol the parks. They claim, in their two years on patrol, to have saved the life of a transit policeman, intervened in countless muggings, deterred crime, and made 128 citizen's arrests. They've now got branches in Los Angeles, Philadelphia, Hartford, Conn., Newark, N.J., and San Juan.They're looking to expand to Chicago, Boston, Dallas and Washington.

Despite this, for most of their existence they've inspired mixed feelings in their hometown. Mayor Edward Koch said that people truly interested in doing good do not try to get on television. Sliwa -- who had sold his story to CBS and is perhaps the only gang leader in the country with a publicist -- responded by calling the mayor a "bonehead."

But sometime last fall, the party line changed. The mayor appointed his criminal justice coordinator to investigate the Guardian Angels, and that report was so positive that the Guardian Angels will soon be awarded some sort of official status.

Which is not to say that feelings among New Yorkers are 100 percent pro-Angel. They are, in fact, more or less divided along class lines. Ask a nicely dressed gentleman from Manhattan's East Side, sitting in the well-traveled Bloomingdale's subway stop, and he'll give the Angels guarded, somewhat cynical praise.

"Noble intention, I suppose," he'll say, watching the much photographed kids, in their jeans or kung-fu costumes, being photographed some more.

But ride the Mugger's Express, and you'll see something else: families, single young women, old men with accents coming up to the kids, or to Sliwa, smiling, asking for an autograph, shaking his hand.

Thank you, they mostly say.

The leader of the Guardian Angels, the man who put it all together in the first place, is Sliwa, a former assistant manager of a Bronx McDonald's. He is known, in Guardian Angel circles, as the Rock, and he's a celebrity these days, appearing on Tom Snyder's television show, strolling though Greenwich Village with Abbie Hoffman, traveling to Philadelphia for a radio show.

Days he makes public appearances, nights, he commands his patrols from the Lexington Avenue subway beneath Bloomingdale's. If you want him at night, the best place to try him is the pay phone in the subway stop where he usually takes his calls. He also places calls to public phone booths, calling patrol leaders in Brooklyn, the Bronx and New Jersey.

Delegating authority is not his thing.

He wants what he wants when he wants it. He needs his boots, so an adoring Guardian Angel will be dispatched on a 40-minute trip uptown. The adoring kids, usually much younger than the Rock, less educated, don't mind.

"To me, he's like a father," says 17-year-old Kool-Aid, carrying the boots in a plastic bag. "He's so great. Ya got any problems, ya can go to him. It's like a family. Better than a family."

A tough family, though. An Angel stands at attention and is frisked for weapons before going on patrol. The Angels, men or women, walk to trains in single file.

On the train, each Angel takes a car, and signals to the next, car to car, with a wave of the beret, or a sign, at trouble. An Angel is suddenly not at his post, and within seconds the rest of the patrol is in his car, to back him up.

Without weapons, with often only rudimentary training in the martial arts, the Guardian Angels control and disarm through numbers: a minimum of eight people on each patrol. The only time an injury occurred, in Angels history, was in the early days, when Sliwa, on a three-man patrol, tried to stop a rape and was pushed from an elevated platform.

Get into a fight on a patrol, start kidding around, and the patrol leader or Sliwa will strip you of your colors, taking away your Guardian Angel T-shirt or beret.

"You got to be perfect," Sliwa tells his troops. "You're up on a pedestal. A cop messes up, there's still cops. There's Abscam, but you still have Congress. But a Guardian Angel messes up and it comes down on everybody. We can't have a bad Angel."

Sliwa, as both he and his family tell it, was always a good little angel, from a background very different from the ghetto kids who comprise his group.

He was middle class, born in Brooklyn to adoring parents. At 5 he read, at 7 he studied piano and karate. But perhaps the greatest influence, as Sliwa tells it, and he tells it often, was his immigrant Italian grandfather.

"He instilled in me my feeling for volunteer service," says Sliwa, who likes to impress the troops with big words. "He gave me a lot of my value system. He was 70 years old and bent over, and he'd still sweep the sidewalk in front of the house every day. He taught me two things. 'Don't be afraid of hard work' and 'Don't wait for anybody else to do it. Get off your butt and do it yourself.'"

The morality lessons took. As a 5-year-old, grocery shopping with a neighbor, Sliwa loudly reprimanded the woman for offering him some candy before she came to the checkout line. "You opened it up and you didn't pay for it?" he yelled. "That's stealing!"

At 16, according to his mother, he apprehended a stickup man in a grocery store. "He took the criminal and flung him out the window," says Fran Sliwa, proudly. "He was always very conscious of right and wrong."

He was also independent. Thrown out of high school in a dispute over the dress code -- with, he claims, Ivy League scholarships waiting -- he never returned.

Then, in 1977, making the long commute from the Bronx McDonald's where he worked to his home in Brooklyn on the Mugger's Express, the idea for the Guardian Angels began.

He rode the subway line, accompanied by a large, strong friend called the Chinaman. He decked himself out in a three-piece suit, carried an expensive radio and appeared to doze. ("I looked like a big turkey, and anyone with evil on his mind would gaze at me and see turkey, giblet gravy, and all the stuffing," he told one reporter.) When the inevitable happened, and he was mugged, he signaled the Chinaman with the beepers they both wore.

From there, it was just a short jump to Sliwa's first group: the Magnificent Thirteen, which, three-man patrols, proved too small for safety. Now, Sliwa figures, in 10 years the group will be national, international.

Talk to other Guardian Angels, many of them teens, most with fewer advantages than Sliwa, and many with so much energy they snap into a karate stance in the middle of an interview, and the attraction the group holds becomes clear.

"Most of the kids in the Angels, they don't get along with their mothers, they don't know who their father was, maybe they had parents who left them," says Lisa Evers, a Manhattan group leader, who's had to fend for herself through her teens.

"Maybe they've never been in a house, never seen an adult male who didn't hurt someone else. They have no concept of something positive. They see the straight world, the 9 to 5, and they think those guys must be suckers to work like that for 10, 15,000 a year, when the guy selling drugs on the corner can make a thousand, 2,000, in a night. They're young people, they're fine young people, but they need some direction. And we give them that. We show them it's cool to be good, it's cool to be positive."

Rocky, 21, also a group leader, echoes that. "I was in a gang in Chicago, a gang in Queens. I wasn't doin' so good, I left home when I was 15," he says. ". . . The problem of not being wanted by my family . . . never got into big trouble, mostly misdemeanors, used to do graffiti -- maybe rip somebody off . . . nothing serious, but I wasn't goin' nowhere. . . . Then a good friend of mine starts talking to me about the Guardian Angels. . . . He knows I always had a basic goodness, how I'd go help, I see something going on in the street. . . ."

Now he runs the biggest Angel patrol in the city. He can't say he's stopped any serious crimes, but he feels he's deterred a lot.

"I'm not coming on like a cop. I'm not tryin' to make a quota," he says. "I see some 13-year-old messing up my train, I'm not gonna arrest him. But I'll let him think for awhile I'm gonna arrest him. I'll scare him a little, I'll say 'Okay, man, I know what you're doing. I used to do it myself. . . . We're like you. . . .'"

A regular tough-guy good guy. A kung-fu kicking, mean-but-clean, down-to-the-ground ghetto Boy Scout.