Mayor Lori Lightfoot said Thursday she made the unprecedented decision to shut down Chicago’s most popular gathering spots — including the entire lakefront — to prevent the coronavirus tragedy now unfolding in New York City.

One day after warning Chicagoans of the potential consequences of defying a statewide stay-at-home mandate, Lightfoot pulled the trigger on a public health order that cuts off access to the lakefront and all its parks and beaches, along with Millennium Park, the downtown Riverwalk and the 606 Trail.

Everyday life in Chicago — already changed immeasurably for people laid off from work or forced to work from home — will change even more. For now, Chicagoans with cabin fever have lost access to the outdoor places that gave them comfort.

Lightfoot was forced into a policing role she never wanted after seeing “crowds of 100 or more” over the past few days “congregating together, particularly along our lakefront and along the 606 and other places.”

It’s a “blatant violation” of Gov. J.B. Pritzker’s stay-at-home order imposed just a week ago, though it seems like years.

“Your conduct — yours — is posing a direct threat to our public health. And without question, your continued failure to abide by these life-saving orders will erase any progress that we have made over the past week in slowing the spread of this disease and could lead to more deaths,” the mayor said.

“If you don’t act responsibly and stay at home like you have been ordered to do, we will be headed for a situation like we’re seeing play out catastrophically every day in New York. This will push our city to the brink. … Congregating on our lakefront, to be blunt, is going to create a risk that is unacceptable and could lead to death. That is why we are taking these actions and going back and saying again, ‘Dear God, stay home. Save lives.’”

Lightfoot said the closure will remain in effect “until the public health commissioner determines that this threat of our lives is over.”

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She has directed the Chicago Police Department to aggressively ramp up patrols in all of the now-shuttered areas. They’ll start with a warning, followed by a ticket. Defy both, and “we will arrest you,” the mayor said.

“Folks, we can’t mess around with this one second longer. We’ve seen around the world example after example of what happens when communities don’t take this threat seriously.”

Although the closings are concentrated along the lakefront and in the downtown area, Lightfoot stressed the order is not an invitation to gather “in other parks and playgrounds” away from the lakefront.

Lightfoot denied that the edict was tantamount to “martial law,” arguing that it was vetted by city attorneys “twenty ways from Sunday.”

She acknowledged it “may seem extreme to some people,” but said she “won’t hesitate to take any further action needed” if it’s warranted by “the science” and if we need to do more to reinforce for those of you who have failed to get the message” not to congregate.

Normally, the American Civil Liberties Union of Illinois would oppose the blanket closing of Chicago’s most popular gathering spots. Not this time.

“Let’s see how this is enforced. Let’s see what this looks like in a week. But if decisions are made on the basis of advice from public health officials to try to address this pandemic situation, those are things which are likely to be permissible,” said ACLU spokesman Ed Yohnka.

“We’re living in an unprecedented time of a pandemic. There are going to be limitations on our movements as a result of that. It’s a step that was taken that, perhaps, will make people pay attention and, maybe at the end of the day, doesn’t last or is necessary for a protracted period of time, but sends a signal about what’s important.”

Downtown Ald. Brian Hopkins (2nd) applauded the mayor’s “difficult, but necessary” decision, saying it was based on “reports from the field” over the last 48 hours.

“It wasn’t just the sheer volume of people congregating together, but reports from Chicago police officers regarding the complete lack of cooperation when they would tell people, ‘You need to go home.’ People just weren’t taking it seriously. They weren’t leaving the park. Or if they were, they would leave the park for two minutes. So the police moved along and they would come back,” Hopkins said.

Hopkins said there is “no doubt that, in the past 48 hours, people got infected in the parks and on the lakefront trail.” He noted people running, jogging, biking, playing basketball and soccer perspire more, breathe more heavily and infect each other when they come in contact or get too close.

“This really is a matter of life and death. That’s not an exaggeration. To the extent that we can engage in social isolation now, it’s going to cut down on the number of peak infections that have the potential to overwhelm our health care system. This is the moment we needed the most cooperation and we just weren’t getting it,” the alderman said

Lakefront aldermen Harry Osterman (48th), Michele Smith (43rd) and Sophia King (4th) were equally supportive.

“The residents of my lakefront community will adjust to this change as they have been adjusting to other changes to their daily lives,” Osterman wrote a text message to the Sun-Times.

Smith advised those chafing at the mayor’s order to “Google what’s happening in New York” and think about how much worse things could get in Chicago.

“It’s really unfortunate that it needs to be done. But, every person ... inadvertently having contact with someone could be another person catching the disease,” Smith said.

“We have seen too many people endangering themselves, probably unknowingly or disbelievingly, that, if they are with people that they know, how can they really get sick?”

Although south lakefront crowds were much smaller than on the North Side, King, whose 4th Ward covers part of the south lakefront, said the order is justified.

“People are not acting responsibly. ... Because individual irresponsibility affects the whole, we have to act aggressively to mitigate the risk for everyone,” she wrote in a text message.

“We are on a very concerning trajectory. If it continues to increase at this rate, hospitals will be overwhelmed. This pandemic is serious! People need to start acting like it is!”

Juanita Irizarry, executive director of Friends of the Parks, said she is “comfortable with some closures” along the lakefront, but would prefer “measures that address trouble spots,” instead of a blanket closure.

“I don’t know if the South Side lakefront was as busy as the North Side lakefront” on Wednesday, Irizarry said.

Although closing the entire lakefront is unprecedented, Irizarry is not accusing the mayor of going too far.

“I do think we need places to walk and run and be healthy. But if we can’t use that space in a healthy way, we do need to make sure people are not congregating,” she said.

“I would say that Friends of the Parks is thankful for a selective approach to closures that does still leave other parks and green spaces open for people.”

Contributing: Mark Brown