Soon tourists who visit Washington, D.C., won't have to worry about bringing cameras. The police will be taking their pictures.

The Wall Street Journal recently reported that the Washington Metropolitan Police Department plans to monitor people through hundreds of cameras on streets, in subway stations, and possibly in shopping malls and other businesses. "In the context of September 11," said Stephen Gaffigan, the official in charge of the surveillance network, "we have no choice but to accept greater use of this technology."

Let's pretend, for the sake of argument, that we do have a choice. Fighting terrorism may have replaced protecting children as an all-purpose excuse for expanded government, but there is still something undeniably creepy about ubiquitous, centralized surveillance of public spaces.

Admittedly, this vague uneasiness, even if it's backed up with references to 1984 and Enemy of the State, is not enough to trump fears of terrorism. So what, exactly, is the problem with using cameras to enhance the police presence in our nation's capital (and soon, perhaps, in your town)?

The problem does not seem to be constitutional. The D.C. police are not peeking into people's homes or car trunks; they're watching people in public, something any citizen has a right to do. Staring at someone on the steps of the Lincoln Memorial may be rude, but it does not constitute a "search" under the Fourth Amendment.

In some respects, putting up a camera is like putting another cop on the street. "We don't have enough officers to watch everything," Gaffigan told The Washington Times. "This allows us to monitor more places and frees up officers to do their work in the neighborhoods."

But do we really want police to "watch everything"? Manpower constraints are not the only argument against putting a cop on every corner.

When police are omnipresent, people are more careful about what they say and do, which has negative as well as positive implications. There may be less noise and less littering, but there is also less debate and less spontaneity.

Even in a country with a tradition of limited government, knowing that you are being watched by armed government agents tends to put a damper on things. You don't want to offend them or otherwise call attention to yourself, so you are not quite as free as you would otherwise be.

When police are invisible but still watching, people conscious of the surveillance will still change their behavior to avoid being conspicuous. Meanwhile, people who don't know about the cameras or who forget about them (a tendency on which the hope of catching criminals hinges) may receive uncomfortable scrutiny even though they are doing nothing illegal.

Nervous-looking swarthy guys--most of whom, needless to say, are not carrying explosives--are not the only potential targets. How will police react to a young man reading the Koran or The Anarchist Cookbook while sitting on a park bench?

After a while, people may learn to be careful about the books and periodicals they read in public, avoiding titles that might alarm unseen observers. They may also put more thought into how they dress, lest they look like terrorists, gang members, druggies, or hookers.

Widespread surveillance can be expected to deter a wide range of potentially embarrassing or provocative activities. Perhaps you'd just as soon see less kissing, nose picking, and pot smoking on the street, but you may feel different about parents who are newly reluctant to discipline their kids in public.

Because they observe people unobtrusively, surveillance systems give police access to sensitive information they could not otherwise obtain. A camera in an officially public but out-of-the-way spot could reveal someone's sexual preference or extramarital affair, putting him at the mercy of police discretion.

Expecting bored officers to resist their voyeuristic impulses is probably unrealistic. In a New York Times Magazine article about the British surveillance system, often held up as a model by U.S. law enforcement officials, Jeffrey Rosen reports that officers use police cameras to ogle women and spy on fornicating couples.

Surveillance advocates insist they'll restrain themselves. According to the Journal, "The police say they are aware of privacy concerns and plan to stop far short of the level of constant surveillance the technology allows."

In other words, they promise to be willfully blind. That doesn't mean we should be.