Pedestrians freeze mid-step and bustling afternoon traffic stops around Union Square as a squadron of about 50 police cruisers pulls away in groups of 10. The sound of all those sirens blaring can make even the most seasoned New Yorkers put down their cell phones and gawk in bewilderment at the chaos of red and blue lights.

There is no crime scene for officers to rush toward, but NYPD commanders say a precise strategy lies beneath the veneer of pandemonium.

This display of manpower is known within the New York Police Department as the “surge” – a daily dose of psychological warfare designed to confuse potential terrorists. Every day, the surge includes different squad cars from all of the city’s 76 precincts and additional units.

Wearing their full uniform, including a gas mask in a cylindrical case attached to their belt, the officers in the Critical Response Vehicle unit are ordered to “combat park” their patrol cars – leaving them perpendicular along the sidewalk against a wall to make their presence more visible. They interview security managers of buildings and subway station attendants rather than simply observe the area.

Each evening, detectives from the department’s anti-terrorism Intelligence Division – which includes detectives permanently stationed in nine foreign countries – select the next day’s “surge” route. The routes are never the same. Sometimes the teams deploy early in the morning, sometimes late at night. And sometimes, the NYPD stages two consecutive “surges” in a row.

The message to potential terrorists is: “This is not the place to go if you are planning a terrorist action,” an investigator from the Terrorist Incident Prevention Unit, Neil Hirsch, said. “We want to stay away from routine,” he said. “We don’t want people to know exactly what we’re going to do on any given day.”

After the cars were gone and the park’s midday rhythm was restored, people walked on their way, unaware of exactly what it was they had seen. The impression, however, lingered.

One man said into his cell phone, “There are a lot more officers on the street today.”

Before heading out at about 3 p.m. on Tuesday, officers were briefed on the most recent intelligence about terrorist activity in New York and abroad. Under the command of a Queens North inspector, Salvatore DiPace, the cars – each carrying two officers – then sped south on Union Square East, before dispersing in groups to their targets. The Critical Response Vehicle unit paved a Manhattan path that passed by key subway stations and signature office buildings, including the Citibank building.

“New Yorkers can be very complacent,” Mr. DiPace said. “We want to wake them up.”

With the terrorist attacks on Egyptian resorts having taken place less than a day before Tuesday’s surge, officers were briefed on the details of those attacks. “I was getting text messages right off the bat about how many injuries yesterday,” Mr. DiPace said.

Like all the eight counterterrorism coordinators in the department, and other ranking officials, he is one of the first to know when a terrorist strikes abroad or when intelligence surfaces about a potential attack here.

In the months following the September 11, 2001, attacks, Commissioner Raymond Kelly came up with the idea for the Critical Response Vehicle strategy during discussions with his top staff about using existing resources to protect New York City. The daily “surge” is part of Operation Atlas, the coordinated deployment of the department’s counterterrorism resources across the city.

In a briefing packet given to officers before each “surge,” there is a reminder that the men who were plotting an attack on the Brooklyn Bridge backed out, saying, “New York City is too hot.”

The NYPD has responded to the growing threat of “homegrown” terror cells – unconnected with international groups – by creating a culture of omnipresence. In the last year alone, the NYPD began installing hundreds of high-tech cameras on street corners and officers began randomly searching bags on subways.