The NYPD is on the lookout for individuals armed with "conventional or improvised weapons" who may unleash a devastating attack on the city by concealing deadly chemicals in light bulbs or fire extinguishers, The Post has learned.
A four-page alert issued by the NYPD's counter-terrorism unit to cops over the weekend warns officers to keep their eyes peeled for "unusual conditions and activities" that may be a prelude to an attack.
The bulletin warns cops should be prepared for the "release of toxic substances in populated and contained environments" like subway stations and sporting venues.
Cops also were told the deadly poisons sarin gas and cyanide could be contained in a list of specific household items -- light bulbs, soda bottles, aerosol spray cans, fire extinguishers, briefcases or mayonnaise jars.
Police officers are to be on the lookout for men who appear freshly shaven with cuts or nicks -- which could indicate a beard has just been removed -- as well as anyone with ill-fitting uniforms or "unfamiliar forms" of identification.
The memo follows action taken by the feds and New York state Friday to boost their terror-threat alert levels one notch to high-risk "orange" -- the second-highest warning on the five-alert scale. Authorities cited intelligence that suggested a growing threat from Al Qaeda.
The Big Apple has been at the high-risk alert level since the 9/11 attacks.
Homeland Security Secretary Tom Ridge said yesterday the latest terrorism alert represents "the most significant" warning since 9/11.
"One of the reasons that we raised it is that we believe the threat has substantially increased in the last couple of weeks," Ridge said.
The feds recommended Americans take basic disaster-preparation steps such as maintaining a three-day stockpile of food and water and buying duct tape and plastic sheeting to seal a house in the event of a chemical or biological attack.
"Just being more ready -- being more prepared -- is a deterrent in and of itself," Ridge said.
Commuters across New York experienced longer waits to enter the city's bridges and tunnels and saw more inspections of vehicles during the first full workday after the feds elevated the national security alert.
Delays at the Lincoln Tunnel during rush hour were three times longer than normal "primarily due to the increased security that's been in place," said Port Authority spokesman Steve Coleman.
Meanwhile, jittery subway riders were worried about a possible underground attack.
"Just keep your eyes open and your fingers crossed," said Andrew Washington, 28, a bank teller from Manhattan, as he got off a No. 4 train at Grand Central station. "That's all we can do."
Bomb-sniffing dogs and an army of cops patrolled the subways as thousands of straphangers made their way to work.