Police officers, outreach workers and other New York City personnel were on alert for homeless, high or drunken people who could be in danger during the region’s coldest weather in 20 years, Mayor Bill de Blasio said Saturday.

Urging all New Yorkers to stay indoors, de Blasio cautioned that everyone should “change their habits” this weekend, with forecasts calling for face-numbing wind chills of 25 degrees below zero.

“This must be taken seriously,” he told reporters in Brooklyn at the city’s Office of Emergency Management headquarters. “It’s not business as usual.”

Just being outside for 10 minutes could put a person at risk for frostbite, said the city’s health commissioner, Dr. Mary T. Bassett.

Bassett said people who are under the influence of drugs or alcohol, or who are mentally ill, may not fully realize the risks of being out in the frigid cold.

At least 62 homeless people went to shelters Friday night and early Saturday under a policy that allows the government to force the homeless indoors if they won’t go voluntarily during frigid temperatures, the mayor said.

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De Blasio urged people enjoying the city’s night life during the cold snap to exercise caution.

“I am saying folks who, you know, go out on the town for example, get drunk, and then spend time outside — you have to be really careful,” de Blasio said.

Police, firefighters and medics will be out looking for such people, he said. “All of our uniformed services are going to be on high alert . . . looking for any situation where someone may be in danger.”

Mindful of wind gusts, officials have ordered construction cranes around the city to be placed in a secure position, de Blasio said. A sudden wind gust is one of the causes being eyed in the collapse of a crane in lower Manhattan this month, killing a pedestrian.