AMPA, Fla., July 3 — Camera shy? Then steer clear of Tampa's nightlife district.

The Tampa Police Department has placed three dozen security cameras with face-recognition software in a downtown district popular with locals and tourists. Now, everyone who visits the district, Ybor City, for a burger or a beer runs the risk of having his face digitally scanned and the noses, cheeks and chins checked against a mug-shot database of murderers, drug dealers and other criminal suspects with arrest warrants.

The police have used surveillance cameras in other cities to record and catch criminals in the act. But Tampa's effort is the widest by a police department in this country to fish for criminal suspects in a general-public sea, using this technology.

The makers of the system, the Visionics Corporation of Jersey City, offered Tampa free use of it for a year, in an effort to build a market among municipalities. City officials, who had used a competing system in January to scan the crowds at the Super Bowl for possible terrorists, were agreeable.

"It's a public safety tool, no different than having a cop walking around with a mug shot," said City Councilman Robert F. Buckhorn Jr., chairman of Tampa's public safety committee.

Besides, he added, on a Ybor City street of restaurants, nightclubs and stores crowded with 20,000 people, "your expectation of privacy is somewhat diminished, anyway."

Randall Marshall, legal director for the American Civil Liberties Union of Florida, disagreed, saying it amounted to subjecting the public to a digital lineup.

"This is yet another example of technology outpacing the protection of people's civil liberties," Mr. Marshall said. "It has a very Big Brother feel to it."

The system, in effect since Friday, has not yet identified any suspects, though the system used at the Super Bowl identified 19 people thought to be wanted on outstanding warrants for misdemeanors.

Reaction in Ybor City was mixed.

Jason Skinner, a security guard buying sandwiches at a deli across the street from a camera mounted to a utility pole, said that, despite his occupation, he opposed the digital peeping by the police.

"It's invading people's privacy," Mr. Skinner said of the camera aimed in his direction. "They're all over the place."

But some Ybor business owners said they hoped the cameras became a permanent fixture, much like street lights.

"I don't find it an invasion of my privacy, and my customers don't either," said Jill Wax, 48, the longtime owner of La France vintage clothing store and the former president of the Ybor City Chamber of Commerce. "My only problem with it is how many people will they catch, how beneficial will it be? I guess we'll have to wait and see about that."

Since the system, called FaceIt, started, police officers in a nondescript command center in a neighborhood building, monitored a bank of television screens filled with faces in the crowd, zooming in on individuals and programming the equipment to scan them.

The FaceIt computer broke down each facial image into something similar to a map, with 80 reference points to check. If the system matched more than a dozen of those points against an image in its database — which is to include 30,000 faces when it is fully operational next month — it would signal a match.

At that point, a system operator would determine if the images were similar enough to radio a uniformed officer, who would investigate and possibly make an arrest.

No suspects were identified today, though several people were caught licking ice cream, gulping sodas and stuffing their mouths on their lunch break.

Detective Bill Todd, in charge of the operation, said it was not as if people were on "Candid Camera." The department has placed signs in the area warning passersby that "Smart CCTV is in use," referring to closed circuit television, but signs are not visible from every area in which the cameras operate, and most people interviewed did not know what the message meant.

Similar technology is used by banks, casinos and other businesses. But Samir Nanavati, a partner at the International Biometric Group consulting firm in New York, which advises companies that consider using such technology, and others said that people generally expect some form of surveillance in those settings and can decide ahead of time whether they want to be on camera.

"The question is, can they educate people in that area sufficiently enough so that they understand what is taking place?" Mr. Nanavati said. "And, if they do understand, is there any way for them to opt out and choose not to utilize the technology — and the answer is no."

The Tampa police call the privacy issue overblown because the camera does not record images of people who have not been charged with a crime. "We are not cataloguing a thing," Detective Todd said.

"If the image the camera takes is not in the database," he said, "it immediately dumps the image."

Both the manufacturer and the police say the chance of a false arrest based on the facial scanning is slim and an acceptable trade-off for the possibility of nabbing a criminal who might otherwise remain at large.

"We expect the police to see results," said Joseph Atick, president and chief executive of Visionics. "For criminals who object to it or have a warrant out there will be a deterrent factor from people saying `I don't want to mess with this.' "

Detective Todd said that the expenses of operating the system, which would cost about $30,000, were nominal, because the department would use officers who would otherwise patrol the streets.

He said the system let the department "maximize the process of pointing out people we're looking for without putting 20 more officers on the street and looking for those same people."

FaceIt has been used since 1998 in the London Borough of Newham, whose officials have attributed a drop in crime to it. Last month, it was introduced at Keflavik Airport in Iceland.

But Atlanta and other cities have rejected the system because they were not convinced of its effectiveness or accuracy.

Tampa first used face-recognition surveillance — in a competing system — at the Super Bowl at Raymond James Stadium in January. Even though the system spotted 19 people thought to be subjects of outstanding warrants for minor crimes, none were arrested because the crowd was so large and because the number of matches exceeded the police's expectations.

"We thought we were ready to use it, but getting through the crowd and the architecture of the stadium proved overwhelming," Detective Todd said.

The detective said Ybor City had a crime rate about that of the city over all. But the area, he said, had a higher percentage of "crimes of opportunity" — muggings, purse snatchings and the like — in which criminals focus on areas drawing large crowds, making it a good neighborhood to test the system.

Gil Rizzo, 42, an account representative from Tampa relaxing on Seventh Avenue, Ybor's main drag — directly in front of one of the cameras — agreed.

"I'm in favor of it because of the security," he said. "A lot of nights, there has been shoplifting, women got mugged and robbed. It's safer because of the cameras."