Such tactics are used in New York City, where Police Commissioner Raymond W. Kelly (whose department has had armored vehicles for decades) has invoked both the 19th-century military strategist Carl von Clausewitz and the television series “24” in talking about the myriad threats his city faces — both conventional and terrorist. After the would-be Times Square bomber Faisal Shahzad was arrested aboard a plane at Kennedy Airport in 2010, Mr. Kelly calculated the plot-to-capture time: slightly more than 53 hours.

“Jack Bauer may have caught him in 24,” said Mr. Kelly, who served as a Marine commander in Vietnam. “But in the real world, 53’s not bad.”

IN truth, a vast majority of Mr. Kelly’s 35,000-member force are not specialized troops, but rank-and-file beat cops. But that did not stop Mayor Michael R. Bloomberg from sounding like Patton at the Massachusetts Institute of Technology last week, when he boasted, “I have my own army in the N.Y.P.D.,” suggesting his reasons for preferring City Hall to the White House. More disturbing than riot gear or heavy-duty weapons slung across the backs of American police officers is a “militaristic mind-set” creeping into officers’ approach to their jobs, said Timothy Lynch, director of the criminal justice project at the Cato Institute, a libertarian think tank. “It is in the way they search and raid homes and the way they deal with the public,” he said.

The more the police fail to defuse confrontations but instead help create them — be it with their equipment, tactics or demeanor — the more ties with community members are burned, he said. The effect is a loss of civility, and an erosion of constitutional rights, rather than a building of good will.

“What is most worrisome to us is that the line that has traditionally separated the military from civilian policing is fading away,” Mr. Lynch said. “We see it as one of the most disturbing trends in the criminal justice area — the militarization of police tactics.”

Police officials insist they are not becoming more militarized — in their thinking or actions — but merely improving themselves professionally against evolving threats. This is the way to protect citizens and send officers home alive at the end of shifts in an increasingly dangerous world, they say. Of course, in the event of a terrorist attack, they have to fill the breach until federal or National Guard troops can rush in.

“If we had to take on a terrorist group, we could do that,” said William Lansdowne, the police chief in San Diego and a member of the board of the Major Cities Chiefs Association. Though his force used federal grants to buy one of those fancy armored vehicles — complete with automatic-gun portals — he said the apparatus was more useful for traditional crime-busting than counter-terrorism.