“Things the public will see and, of course, things the public will not see,” he said. “All of this will ensure that New York City has a safe and wonderful Thanksgiving, as we do every year.”

As it has for all major public events since the Sept. 11 attacks, the department will have what the chief called a “counterterrorism overlay” to its plans. For the first time, about 200 officers outfitted with heavy body armor and long rifles from the department’s new Critical Response Command, part of its Counterterrorism Bureau, will be on patrol, Chief O’Neill said.

Behind the scenes, the department will be mining its network of global contacts for all the intelligence it can glean.

After the Nov. 13 terrorist attacks in France by the Islamic State, also known as ISIS or ISIL, the department sent a French-speaking detective to Paris to join the one assigned there full time, said John J. Miller, the department’s deputy commissioner for intelligence and counterterrorism. Those officers are working with other New York City detectives in Antwerp, Brussels, London and Madrid, he said, adding that, so far, what the department has learned about the Paris attacks has affirmed its approach to counterterrorism.

“The first thing you see from Paris is these are people who included foreign fighters, who are hardened, trained and experienced,” Mr. Miller said. “They weren’t gathered up by Twitter chats, online, in Paris; they were sent there with a specific task and targets, likely from ISIS command and control, and it was a well-organized plot.”