On the heels of breaking up an alleged bomb terror plot, New York is planning to place high-tech security cameras, license plate readers, and "weapons sensors" in midtown Manhattan.

Office workers and tourists – and possible terrorists – will have cameras watching their every move as they visit Macy's, shop for diamonds at Tiffany & Co., or gawk in Times Square. The apparatus, paid for by some $24 million in Department of Homeland Security funding, will expand a similar effort already underway in lower Manhattan where cameras focus on the Federal Reserve, the New York Stock Exchange, and the Brooklyn Bridge.

Mayor Michael Bloomberg, announcing the program Sunday, said the goal is to detect terrorism threats and deter pre-operational surveillance. Sensors will try to detect chemical, biological, and radiological threats.

But some terrorism experts have questioned whether a camera network will deter terrorists. They also say that sensors are known to give off "false positives."

Meanwhile, civil rights organizations are concerned that the project will be another encroachment on civil liberties.

"The fear is [that] the NYPD without any oversight or public scrutiny is creating a massive surveillance system, when we don't know if this is the best use of $125 million designed to keep us safe," says Donna Lieberman, executive director of the New York Civil Liberties Union (NYCLU). The NYCLU has filed two lawsuits to try to get more public information about the program.

In March, New York Police Department (NYPD) commissioner Raymond Kelly indicated in testimony before the City Council that 1,000 police officers are involved in anti-terrorism work daily, and that he hoped to add 500 more cameras to the 500 already installed.

But there did not appear to be any discussion at the hearing about whether the effort is worthwhile or what kind of civil rights safeguards might be needed.

"There are legitimate arguments on all sides here," says Frank Cilluffo, head of George Washington University's Homeland Security Policy Institute in Washington. "What I would like to see is a broader discussion that brings in the average citizen."

Probably the largest use of security cameras is in London, which has put what it terms a "Ring of Steel" of thousands of security cameras all around the city. But some security experts question their effectiveness.

"They won't stop any terrorist," says Bruce Schneier, chief security technology officer at BT (formerly British Telecom) and a widely-published author on security. "None of them is going to look at a camera and say 'I better go get a real job.' "

The only time cameras reduce crime is in parking lots and laundromats, Mr. Schneier says.

"It only makes sense if the tactics and targets are few, but there are billions of targets ranging from shopping malls to restaurants and dozens of tactics," he adds.

However, Brian Jackson, associate director of the Homeland Security Research Program at the Rand Institute in Washington, says the cameras create "the possibility for prevention" of an attack. He says cameras add a level of concern for the terrorists.

"Surveillance is one ingredient that gives the terrorists more opportunities to make a mistake and be discovered," says Mr. Jackson.

As for using sensors that detect biological and radiological weapons , Mr. Cilluffo says they often give off false positives or false negatives. "They tend to be only successful in events that can easily be contained like the State of the Union address or conventions," he says. "Once they are in a more dynamic environment, I'm not sure where the science is on that."

But the science of cameras is progressing fast, raising the possibility of even more "Big Brother"-like scenarios.

"Cameras are everywhere and you can see them," says Schneier, "In ten years, you won't even be able to see them."

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Read more about how New York started the surveillance system in lower Manhattan in 2007.

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