NEW YORK — Sand-filled sanitation trucks and police sharpshooters will mix with glittering floats and giant balloons at a Macy's Thanksgiving Day Parade that comes in a year of terrible mass shootings and not even a month after a deadly truck attack in lower Manhattan.

New York City's mayor and police brass have repeatedly stressed that layers of security, along with hundreds of officers, will be in place for one of the nation's biggest outdoor holiday gatherings, and that visitors should not be deterred.

"We had a couple of tough months as a nation," Police Commissioner James O'Neill said. "We won't ever accept such acts of hate and cowardice as inevitable in our society."

A posting last year in an English-language magazine of the Islamic State group, which took credit for the Oct. 31 truck attack that killed eight people, mentioned the Thanksgiving parade as "an excellent target." Authorities say there is no confirmation of a credible threat.

"I want to assure the people that we swore to protect that anytime something happens anywhere in the world, the NYPD works with our law enforcement partners and studies it and we learn from it and it informs our decision making going forward," O'Neill said.

This year's security plan includes dozens of city sanitation trucks, which weigh about 16 tons empty and up to twice that with sand, that will be lined up as imposing barriers to traffic at every cross street along the 2 ½-mile parade route stretching from Central Park to Macy's flagship store on 34th Street.

In addition, officers with assault weapons and portable radiation detectors will walk among the crowds, and sharpshooters on rooftops will scan building windows and balconies for anything unusual.

Mayor Bill de Blasio said Wednesday that security at the parade increases every year "because we understand we are dealing with a very challenging world." He told crowds gathered to watch the parade's giant balloons being inflated that "the amount of resources and personnel we put in has increased each year to make us safer."

New York officials also are asking the tens of thousands of spectators to be alert for anything suspicious.

"There will be a cop on every block," said NYPD Chief of Patrol Terence Monahan. "Go to that cop and say something."

The 91st annual parade begins at 9 a.m. and will be broadcast live on NBC. Smokey Robinson, Jimmy Fallon, The Roots, Flo Rida and Wyclef Jean will be among the stars celebrating, along with performances from the casts of Broadway's "Anastasia," ''Dear Evan Hansen" and "SpongeBob SquarePants."

New balloons added this year include Dr. Seuss' Grinch, Olaf from the smash movie "Frozen," and a puppy called Chase from Nickelodeon's "Paw Patrol."

Beyond the pageantry, police say they have been working on security for the parade since the moment last year's parade ended. It's a plan that got renewed attention after a terror attack in lower Manhattan Oct. 31, when a man in a rented truck barreled onto a crowded bike path near the World Trade Center, killing eight people.

Authorities said the 29-year-old suspect operated from a playbook put out by the Islamic State group. Sayfullo Saipov, an Uzbek immigrant, was charged with federal terrorism offenses that could qualify him for the death penalty. According to a criminal complaint, he made statements about his allegiance to the Islamic State group.

The first major event since the attack — the New York marathon, which drew tens of thousands of spectators and 50,000 runners from around the world — went off with no problems.

"We said right away New York's response is to remain strong and resilient," de Blasio said. "We do not back down in the face of terror threats. The city is filled with resolve."