New microphone technology can isolate the sound of a gunshot and direct the attached camera to swivel and zoom in on the source. Sensitivity may reach the point where microphones could pick out the word "explosives" spoken in a crowd.

"There's just not enough personnel to watch every single camera," said Chicago emergency operations chief Andrew Velasquez. "We are piloting analytic software right now ... where you can set that particular camera to watch for erratic behavior, or someone leaving a suitcase on the sidewalk."

Since the attacks on the United States of Sept. 11, 2001, sections of New York, Washington, Los Angeles, Chicago and even a few smaller U.S. towns have been blanketed with closed-circuit cameras. Privately owned cameras are also proliferating.

The encroachment on privacy in what civil libertarians call a "surveillance society" may be a price willingly paid by citizens who fear terrorism and crime.

But ever-alert software capable of maintaining a continuous "watch" on security cameras multiplies the risks of harassing innocent people, privacy experts say.

"I don't buy it. The number of false positives are going to be astronomical," said David Holtzman, author of "Privacy Lost." "It's extremely dangerous to abrogate legitimate law enforcement authority ... to a camera."

In Chicago's darkened, windowless surveillance center, Velasquez looks forward to using new technology, which has had some success elsewhere.

The port of Jacksonville, Florida, has dispensed with human monitoring of cameras altogether by sending alerts and live video to the personal digital assistant of the nearest officer on patrol, according to a spokesman for ObjectView Inc.

ObjectView is one of two dozen companies seeking to perfect so-called intelligent video -- an industry whose sales will grow from $60 million to $400 million within five years, according to global consulting group Frost & Sullivan.

Meanwhile, Texas is evaluating a pilot program in which it allowed Internet access to video of unmanned sections of its border with Mexico and urged viewers to send an e-mail if they spotted something.

"The cameras don't replace police officers. They are in essence a force multiplier. They serve as an extra set of eyes," Velasquez said.  

OGLING

The Chicago center is manned 24 hours a day by veteran police officers. A dozen screens depict a few street corners and a stadium, while others are tuned to cable news or Web sites.

They can retrieve video from thousands of cameras and their universe is expanded by private cameras owned by cooperating buildings and stores, but they can monitor only a few at a time.

Velasquez said his officers receive training on privacy and constitutional rights -- for example it is illegal to look into private homes and offices -- and digital recordings hold his officers accountable and prevent abuses that have occurred elsewhere.

In Britain, which has 4.2 million government security cameras, 2 million in London alone, a study showed that male surveillance workers sometimes ogled women on their screens, while others focused on minorities excessively.

But privacy experts also note another British study, from 2002, which said surveillance cameras did not lower overall crime rates, and mereley pushes crime elsewhere.

"Cameras are great tools for solving crime. They're not really that helpful in preventing crime," said Ed Yohnka of the American Civil Liberties Union.

Velasquez disputed the conclusion that cameras don't prevent crime, saying he constantly fields requests from residents asking for a camera to make their neighborhood safer.

He said cameras contributed to a drop in violent crime in the city of Chicago in recent years, a drop that is widely attributed to improved police work in countering gangs and street-corner drug dealing. At the same time, gang activity has surged in some Chicago suburbs.

The city's prosecutors said they rarely use video evidence in court from the cameras, which are encased in bulletproof boxes topped by blue flashing lights and are a common sight in crime-ridden neighborhoods.

Downtown, the cameras are less obtrusive, though a pair mounted on a park fountain was removed after an outcry that they defiled the art.

Holtzman, the privacy expert, wondered where the line will be drawn if authorities opt to use the cameras to spy on suspects or to sniff out low-level crimes.

There are no legal barriers to video being subpoenaed by, for instance, a divorce lawyer seeking evidence of infidelity, he said.

"I think there's a certain amount of freedom you want to give people that live in the city to kind of screw up a little bit," he said.