The national president of the Fraternal Order of Police demanded Thursday that Mayor Lori Lightfoot apologize to rank and file Chicago Police officers — and “conduct herself with more dignity and less immaturity”— after Lightfoot was captured on an open mic calling their second vice-president “this FOP clown.”

Chuck Canterbury jumped into the controversy triggered by Lightfoot’s off-handed remark, first by tweeting the demand for a mayoral apology, then by accusing Lightfoot of immature behavior that has left Chicago Police officers feeling unappreciated by their new boss.

“She’s not off to a very good start. ... She has not demonstrated a level of professionalism that she’s demanding from her employees. ... She basically needs to re-boot somehow,” Canterbury told the Sun-Times in a telephone interview.

“They’re professionals. They understand that she’s the mayor. But, they also believe that she needs to listen more and talk less. … She’s surely not endearing herself to the people that are paid to protect the city of Chicago. If you continuously degrade people, it effects morale greatly.”

Just over two months into a four-year term, Lightfoot has alienated a police union that didn’t trust her to begin with, thanks to her days as Police Board president and co-chair of the Task Force on Police Accountability.

The first insult was the new mayor’s decision to choose retired U.S. Marshal Jim Smith to head a bodyguard detail that, for every other mayor, has been run by Chicago police officers.

Then came her decision to repeat publicly an admittedly “unsubstantiated rumor” she claims to have heard from a “credible” source: that the local FOP had instructed its members to “lay back” and “do nothing” over Memorial Day weekend.

“It was just totally fabricated. We don’t know where she heard it or whether she made it up,” said Canterbury, President Donald Trump’s nominee to head the Bureau of Alcohol, Tobacco, Firearms and Explosives.

The blame game continued with Lightfoot’s decision to mix it up with the FOP’s second vice-president Patrick Murray on the floor of the City Council in June, then publicly shame First Deputy Superintendent Anthony Riccio for taking a family vacation to Aruba during the first week of June. The trip had been authorized and paid for in October, but it defied the mayor’s edict that no top brass take time off during the summer.

That was followed by the open mic embarrassment at Wednesday’s Council meeting, also involving Murray.

Lightfoot was overheard telling her corporation counsel, “Back again. This is this FOP clown” as Murray rose during the public comment section to defend the four Chicago Police officers fired by the Police Board for covering for Jason Van Dyke, the officer convicted of second-degree murder for shooting Laquan McDonald.

Afterwards, Lightfoot said she was “sorry that I said that out loud,” which the Chicago FOP didn’t consider an apology.

The police union branded the remark “contemptuous, misguided and dangerous,” particularly when Chicago is “facing such chronic violent crime.”

Canterbury said Lightfoot “owes” Murray a personal apology — and, if she is “truly sorry, she needs to do something to improve her relationship” with rank-and-file Chicago police officers, whose union contract expired more than two years ago.

“She needs to bargain in good faith rather than make comments like she made on that open mic. I’m sure she’s sorry she said it on that open mic. But an act of contrition needs to be followed by positive action,” he said.

As for the consent decree that Lightfoot has accused the local FOP of obstructing, Canterbury said it was “rammed down their throat” by now-retired Illinois Attorney General Lisa Madigan, “who had promised input from the rank-and-file, and then reneged on her promise.” Madigan has denied that charge.

“Consent decrees have shown to be very ineffective across the country. Collaborative agreements, where everybody sits down and works towards a common goal, have shown promise. Like in the city of Cincinnati,” Canterbury said.

The mayor’s office had no immediate comment on Canterbury’s comments and his demand for a sincere apology.

Chicago Police Superintendent Eddie Johnson has been summoned to weekly “accountability” meetings with Lightfoot after every violent summer weekend.

Asked Thursday to weigh in on Lightfoot’s “clown” comment, Johnson said: “I’m gonna stay out of that fight. It’s just ... For all of us, we just have to work together.”

Contributing: Sam Charles