For the first time since his beatification, Rudy Giuliani is setting forth to resume his battle with the forces of darkness.

They await him now in Mexico City. In that immense megalopolis of about 17.9 million human beings, all citizens, rich, poor and middle class, believe that crime is out of control. Each year, day and night, hundreds of thousands of citizens are assaulted and robbed on the streets. Drug use is escalating among the young.

At least 80 youth gangs impose their will on the poorest, hardest-working citizens. Kidnapping is frequent, including a specialty called kidnapping express, where the victim is taken to an ATM to make a terrified withdrawal. Murder rates are climbing.

"I stopped going out at night 10 years ago," one Mexican friend told me yesterday. "Now I'm afraid to go out in the daytime."

Enter Giuliani.

Like him or not, his record as New York mayor in the fight against crime is the envy of many cities, Mexico City among them. Those other cities are drawn to his insistence on zero tolerance for all crimes, large and small. They are in awe of the overall 57% drop in crime on Giuliani's watch, and the 67% decline in homicides. Now, a group of Mexico City businessmen has agreed to pay $4.3 million in privately raised funds to the Giuliani Group for a year-long study of their unraveling city and the creation of a plan for drastic change. Giuliani clearly feels he is up to the challenge.

"Back in the early 1990s," he said, "New York City was regarded as the crime capital of America, featured on the cover of Time magazine as a rotting apple. Mexico City faces a challenge like that today."

His private consulting firm (including former Police Commissioner Bernard Kerik) has the full support of Mexico City's left-wing mayor and will work with the city's new police chief, Marcelo Ebrard. They've pledged full cooperation.

"Sure, there are differences between New York City and Mexico City," Giuliani said, "but I'm not sure those differences are relevant to crime reduction."

Compstat vs. corruption

The most crucial difference involves reporting crimes. The famous Compstat system is based on analysis of reported crimes. If there is an increase in rapes or assaults in a given precinct, more cops are called in to capture the criminals and smother the crime wave.

But Mexicans of all classes believe that their police forces are hopelessly corrupt, with many cops actively functioning as professional criminals. In the past decade, hundreds of police officers have been arrested as bank robbers, kidnappers, burglars and drug dealers. Today, it's estimated that only 10% of the city's crimes are reported to police, because citizens feel they are wasting their time or, worse, making themselves vulnerable to additional crimes.

"If your home has been burglarized," one writer friend said, "you simply don't want the police to come for a visit. They can learn what kind of security system you have, if any. They can get a sense of your daily routines. How do you say it? They can case the place."

For several years, Mexico City has been trying to better the quality of its police, and there are certainly some good honest police officers working in the city. But often, the corruption begins very quickly. Two years ago, a teacher at the Mexico City equivalent of the Police Academy was caught taking bribes to pass probationary cops into the force. That is, he was corrupting them before they were even sworn in and handed badges and guns.

The resulting cynicism was surely reinforced when those young cops went on to precincts where commanding officers took bribes from drug dealers, pimps, peddlers of contraband. Or, as is too common, extorted money from the young cops themselves to ensure good assignments and future promotions. For many in Mexico City, a war on crime would be a war on the police.

But cero tolerancia also includes quality-of-life offenses, and this, too, would present a mammoth challenge to Giuliani's planners. When I was a student in Mexico City in the 1950s, it was the most beautiful city I'd ever seen. Today, it's the world capital of vandalism.

Gobs of graffiti

Virtually every building on main thoroughfares has been marked by graffiti. Brainless spray-painted calligraphy covers the walls of private homes and public offices, public sculptures and baroque stone monuments. It makes the South Bronx of the late 1980s look like Versailles.

Giuliani has no power to do anything in Mexico City — he's only an expensive consultant. But I'm among those who want him to succeed in helping Mexicans save their brutalized capital city. I have friends who live there, trying to do their work and get through another week alive. Each day, they walk under greasy clouds of corruption and menace, and they deserve neither.

In his often rude way, and with much luck, Giuliani succeeded in lifting the sense of menace from the streets of New York. That, plus his performance Sept. 11, has transformed him into a kind of myth. In Mexico, he must convince the cynical that better times are possible. He has to convince them that the myth is not a lie.