“Gentlemen, start your engines,” NYPD Commissioner William Bratton said from an elevated platform.

Then, as Bratton waved a black and white checkered flag like those used at speedways, two heavy bulldozers moved forward to crunch, grind and otherwise destroy dozens of illegal dirt bikes and ATVs confiscated during the latest police push against what police called a public menace of illegal motorized vehicles.

The mass execution of 70 bikes, which took about two minutes, is one of a number scheduled at the NYPD’s Erie Basin auto pound in Brooklyn as part of the department’s highly publicized push against illegal bikers who Bratton said had terrorized neighborhoods.

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“We want to send out a very strong message to the nitwits and knuckleheads who insist in operating these illegal vehicles on the streets, sidewalks, parks and housing developments of the City of New York,” Bratton told reporters after the destruction.

“They are illegal, they cannot be registered to be used anywhere in the city of New York and thus they are now in the junk heap,” said Bratton, gesturing to the long pile of destroyed ATVs and unregistered motorcycles behind him.

Bratton stressed that lawful riding by licensed drivers of registered motorcycles on city streets is legal. But 80 percent of the 679 vehicles seized so far this year and brought to the pound as part of Mayor Bill de Blasio’s “Vision Zero” accident reduction campaign remain unclaimed, he noted. Last year, at this time there were five fatalities from illegal motorized bike use but this year none, officials said.

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As part of the city offensive, the NYPD is coordinating activities with law enforcement officials in Nassau and Suffolk counties to assure that illegal bikers don’t simply slip over county lines to continue their antics, Bratton said.

Chief of Patrol Carlos Gomez said of the 679 confiscations so far in 2016, 182 were in the Bronx and 108 in upper Manhattan, two hotbeds of illegal biking activity, which generally occurs on weekends. There have been 108 arrests citywide on reckless endangerment charges, he said.

“It can be very intimidating. If you are driving down the public streets or on a highway, these groups of motorcycles pass by you,” Gomez said. “It projects a sense of lawlessness in our city.”

The destroyed bikes, which included some Yamaha and Kawasaki models, will be sold by the city for 2 cents a pound as scrap metal, said Deputy Commissioner Robert S. Martinez, head of police support services. Owners with proof of bikes being legally registered can seek their return, officials said.