But the downtown security plan involves much more than keeping track of license plates. Three thousand surveillance cameras would be installed below Canal Street by the end of 2008, about two-thirds of them owned by downtown companies. Some of those are already in place. Pivoting gates would be installed at critical intersections; they would swing out to block traffic or a suspect car at the push of a button.

Unlike the 250 or so cameras the police have already placed in high-crime areas throughout the city, which capture moving images that have to be downloaded, the security initiative cameras would transmit live information instantly.

The operation will cost an estimated $8 million to run the first year, Mr. Kelly said. Its headquarters will be in Lower Manhattan, he said, though the police were still negotiating where exactly it will be. The police and corporate security agents will work together in the center, said Paul J. Browne, the chief spokesman for the police. The plan does not need City Council approval, he said.

The Police Department is still considering whether to use face-recognition technology, an inexact science that matches images against those in an electronic database, or biohazard detectors in its Lower Manhattan network, Mr. Browne said.

The entire operation is forecast to be in place and running by 2010, in time for the projected completion of several new buildings in the financial district, including the new Goldman Sachs world headquarters.

Civil liberties advocates said they were worried about misuse of technology that tracks the movement of thousands of cars and people,

Would this mean that every Wall Street broker, every tourist munching a hot dog near the United States Court House and every sightseer at ground zero would constantly be under surveillance?