On the Brooklyn waterfront, with the Lower Manhattan skyline visible in the background Tuesday afternoon, two bulldozers rumbled into place on either end of a line of about 70 motorcycles leaning on their sides like a shelf full of half-fallen books.

New York City Police Commissioner William Bratton walked down the line, briefly surveying the bikes, before stepping onto a small riser to launch the destruction with a wave of a checkered flag. Then the bulldozers lifted their scoops and crawled back and forth over the bikes, flattening them with a thrum of metallic clatters and creaks.

"We want to send out a very strong message to the nitwits and knuckleheads who insist on operating these vehicles on the streets (and elsewhere)," Bratton said. They are "creating extraordinary dangers for not only themselves, but for the public."

The department has been cracking down on unlicensed drivers who operate ATVs, mini-bikes and motorcycles without helmets. So far this year, more than 679 bikes have been confiscated and dozens of drivers arrested on such charges as reckless endangerment.

Chief of Patrol Carlos Gomez said nearly 80 percent of the confiscated vehicles remain unclaimed — some are pricey, others are homemade. "And this is their ultimate fate," he said.

The department usually gets about 2 cents a pound from a salvage yard for the wreckage; an auction wouldn't be worth it and there's no telling whether the bikes could be outfitted for legal riding. The idea to crush them came from Bratton's tenure as head of the police department in Los Angeles.

"They're just a plague on the city. We receive a tremendous amount of calls on these things," he said, noting police weren't targeting licensed, law-abiding riders.