"December is mugger's month," declares Guardian Archangel Curtis Sliwa.

"You've got lots of people carrying lots of money and expensive packages in big crowds. You've got people traveling, which means empty houses and out-of-towners. There's more rushing, more intoxication, more carelessness.

"It's a punk's playground. Just like the merchants plan and prepare for their biggest sales at Christmas time, the criminals plan for their best rip-off season at holiday time."

Sliwa, 28, in his tight T-shirt looks a bit like a punk himself, except for the shirt and tie underneath -- "leftovers from my Catholic-school upbringing" in a middle-class Brooklyn neighborhood. The T-shirt and red beret are the official uniform of the Guardian Angels Safety Patrol he founded three years ago in New York "to provide positive role models and give the streets back to the people."

Sliwa's band of tough-looking good guys now number nearly 4,000 in 43 cities (including 48 in the Baltimore chapter). "It's crazy that you have to dress up like a lollipop to let others know you're willing to get involved," he admits. "But it works."

He is now traveling around the country stressing the importance of "street smarts . . . common-sense instincts about danger," particularly at this time of year, and promoting his new book, StreetSmart: The Guardian Angel Guide to Safe Living (Addison-Wesley, 176 pages, $5.95).

"With this book," write Sliwa and co-author Murray Schwartz, 57, the Guardian Angels' chief legal counsel, "we hope to create a movement throughout our country, an outcry, an outpouring of rage and resentment, an organized rebellion by the people against the punks in our city streets."

The biggest myth about street criminals, claims Sliwa, is "that they're professionals. But they're not. Seventy-five percent of the thugs and punks are our own sons and daughters. Like the pirates of old they dress in wild clothes that have a threatening physical look -- mirrored glasses, clashing colors, scarves, studs, anything made of leather.

"They do this to inspire fear, which they thrive on. They want to attract your attention and humiliate you into reacting. Then they have a reason to assault you. They are bullies and cowards who attack those unable to fight back. They are grabbers, snatchers, runners."

Another myth, he says, is that "criminals attack for survival. A lot of time it's the power, not the money. They'll rob bag ladies or hold up someone at knife point for a dollar. You've got bored middle-class kids looking for kicks and poor kids who figure they can't go to school dressed in Good Will. Their idea of survival is designer jeans.

"People have to stop looking out just for themselves and look out for each other," says Sliwa, whose voice takes on a crusaders' intensity when he talks about rallying individuals against crime. "When a street punk violates your neighbor, he is also violating you."

Among his street-smart reminders:

* Always be aware of what's going on around you. "The biggest mistake people make is thinking the safest way to get from point A to point B is moving as quickly as possible. That's the worst thing you can do. It advertises fear and makes you oblivious to what's around you." Instead, "survey your surroundings" and walk "normally and purposefully with eyes open" to your destination.

* Learn to be cool. If you are bumped intentionally by an aggressive person or pestered irrationally over a trivial matter -- such as a parking space -- "smile, talk quietly, agree and exit."

Avoid trouble spots. The edge of the subway platform or last subway car, bus shelters, bus terminal restrooms, dark parking lots and any isolated spot in an urban area.

* Travel in a group if you are afraid to go out alone.

If you see someone being attacked, "shout, scream, attract attention," urges Sliwa. "Do not take on a criminal alone, but arouse your neighbors to join you.

"Many of us are unwilling to assist each other because we believe that in so doing we jeopardize ourselves. At the beginning, that may be true. It takes a unique person to come forth in a confrontation and assist a victim. But if one of you makes the first move, then others will follow."

As an example, Sliwa points to the thousands of people who have followed him -- from "16-year-old kids" (the Guardian Angels' minimum age) to a "72-year-old man in Albuquerque, New Mexico."

Sixty percent of the patrols' growth in the last year has been in the suburbs, he says, with "skate patrols" in San Diego, "bike patrols" in the Midwest and "car patrols" in the Southwest. Patrol members are divided about evenly between blacks, whites and Hispanics. Five out of six are male.

Police reaction to the group, says Sliwa, ranges from "enthusiastic" to "threatened." The Angels have "an official working relationship" with police departments in three cities--New York, Trenton, N.J., and Youngstown, Ohio--where they are issued ID cards and receive assistance in volunteer training. Some cities provide varied support -- "like letting us ride transit systems free."

Plans to start a Washington chapter were scrapped last year after Sliwa was assaulted by four men he claims posed as police officers. Although "Washington is the only place all across the country where we tried to set up a chapter and failed," Sliwa says he hopes to begin organizing here again in the spring.

"We aren't opposed to cooperating with them the Guardian Angels if we can see that their presence here will serve any useful purpose," says Lt. Hiram Brewton of the D.C. Metropolitan Police Department. "If we have a group of youngsters gathering together to do positive activities, that's certainly a positive thing. But as far as hard-core crime, I'd venture to say they'd have little or no effect."

To qualify as Angels, volunteers must be in school, employed or "seriously seeking employment" and volunteer a minimum of four hours a week. They receive three months training in self-defense, law and first aid. None -- including Sliwa -- are paid.

"At first I used the money I made as a manager of a McDonald's," says Sliwa, who travels by bus or by thumb and lives with his wife, Lisa Evers -- the Angels' national coordinator -- in a $155-a-month "dump" on New York's Lower East Side. "Last year my income was $7,500, basically made from speaking engagements."

Why does he do it? "Because of the pat on the back I get everywhere I go. Because it works."