NEW YORK -- When artist Spencer Tunick recruited two women to pose nude by Baltimore's Inner Harbor on a September dawn in 1997, it was no big deal. The women stripped, stood in the tall grass of a field near the Power Plant, and Tunick snapped away.

No muss, no fuss and no nosy police. The photo starred in Tunick's "Naked States" exhibition, and an Austrian connoisseur recently bought it for his private collection.

Tunick, in fact, has staged sunrise shots of nudes in backwater locations from Maine to the Bible belt, assembling up to 1,000 volunteers at a time without stirring a ripple of opposition.

Then how come he suddenly can't do his work anymore in bohemian, anything-goes New York City? Truckloads of police show up every time he tries. Police have arrested him four times and counting, and in April he spent 18 hours in jail, although no court has found him guilty.

Puzzled and embarrassed, Tunick is fighting back in federal court with a pro bono attorney, the flamboyant Ronald Kuby. The result is one of those "only in New York" sagas pitting a free spirit against an inflexible City Hall, a Gotham farce that seems tailored for Hollywood: "Zero Clothing Meets Zero Tolerance,"starring Mayor Rudolph W. Giuliani. Some nudity. Rated R.

"I think the Giuliani administration has a pornographic state of mind," says Tunick, 32, who lives in Brooklyn. "They're only used to seeing nude images in that context. Or in a police strip search. All I'm doing is forming abstract shapes. It is a sea of pink. I'm creating this shape within three minutes, and then it's finished."

Until recently, Tunick had few problems with the law. He liked keeping a low profile, and the only crowds he attracted were those he was shooting. While the city never exactly encouraged his art -- denying a permit whenever he applied -- he was never impeded and only rarely arrested.

"Spencer would get arrested due to bad luck more than anything else," says Kuby, one-time law partner of the late William M. Kunstler, "and I would get the charge dismissed."

Like the time in December 1994, when a Rockefeller Center security guard became irate after Tunick's nude model clambered onto a 12-foot-high Christmas tree ball to pose for a snap. Or like in 1996, just after a big snowfall, when the model posed atop a huge drift next to an ice cream parlor's sign advertising "frozen fantasies."

About a year ago, the atmosphere changed.

At a group shoot scheduled for June 1998 on Delancey Street, the police arrived early, as if invited to the event. Tunick shot the picture and was arrested. At dawn April 25, a Sunday, police arrested him before he could even take a picture. His 150 nude volunteers rose disgruntled from their prone positions on the pavement, got dressed and went home.

That time Tunick spent 18 hours in jail, although the charges were the usual fare: unlawful assembly, disorderly conduct and reckless endangerment.

"You would have thought it was an Al Sharpton political demonstration," says Kuby, whose bushy, graying ponytail flies like a flag of rebellion. "That's when we realized the police were determined to stop him, effectively ending his career in New York City."

Later shoots, scheduled for June 6 and July 11, also never came off -- nor did any clothes -- because police showed up in advance by the dozens with patrol wagons and plastic handcuffs.

Why the crackdown?

At City Hall, Giuliani's press people won't say. Gabriel Taussig, chief of the city's administrative law division, says, "We believe that public nudity of that sort is in violation of state law."

City lawyers have also argued that if Tunick is allowed to get away with it, then what's to stop fraternity boys from proclaiming themselves "artists" before stripping in Times Square?

But no one seems willing to explain what's behind the aggressive pre-emptive assemblies by the police. The rub would seem to be that Tunick has run afoul of Giuliani's new "zero tolerance" policy, with its swift and sure punishment for once-overlooked violations such as littering and jaywalking. It is credited with helping lower the crime rate, even while its excesses may have led to well-publicized cases of police brutality.

"Maybe Rudy Giuliani wants nothing but a G-rated New York." Kuby says. "But of all the things people complain about here, the one thing they never mention is naked people. And when you have a tourist from Iowa, or, I daresay, from Maryland, who sees a naked person in New York, usually they sort of chuckle and chalk it up as one of those Big Apple experiences."

Tunick's work would hardly seem to merit an X rating. In group nude shots, the bodies blend into abstract forms. The 1,000 prone nudes along a Maine riverbank resemble a logjam or a fish kill. A crowd in Central Park looks like waves of grain. In the Nevada desert the bodies weave across the plain like a dry riverbed.

The shoots generally don't create much of a stir, except when truckloads of police arrive. "I make less noise than a city garbage truck, and I work faster," Tunick says. "It's a very simple thing that the city is turning into a First Amendment issue."

On July 16 a U.S. District Court granted Tunick an injunction, on First Amendment grounds, to keep the police from stopping him. The next day, a Saturday, the city got an emergency delay from an appeals panel, which will decide the matter. Next stop: the U.S. Supreme Court.

In the meantime, Tunick may not be able to make any large group shots. The handbills that solicit volunteers inevitably attract police, too. A small shoot may yet be possible. Tunick's friend Kristin Bowler described on a World Wide Web site how the Baltimore photo came about:

"We end up at a cool little vintage store. A woman with cat glasses and blue dreadlocks runs the place. I see lots of stuff to buy, but we have no more money. No more stuff. We ask her to pose and she needs convincing. She's in a band and is recognized around here with that head o' blue dreads and is very cute. Finally she says she'll do it if her friend will. Of course, her friend agrees!"

On Monday Tunick and Kuby were back in city criminal court on the April charges, seated among a gallery of people who'd been arrested for everything from drunken driving to spousal battery.

The district attorney, apparently not yet on board with the City Hall strategy, didn't contest the case, so the judge dismissed the charge. Tunick's not sure what comes next.

"At this point they won't even give me a permit for a single-person nude," he says. "I'm just a face in the crowd. But unfortunately, this is becoming important."