10:46 AM PT -- The National Association of Police Organizations has fired off a letter to Nike chairman and CEO Mark Parker, saying the org. is now calling for a boycott of all Nike products.

"In featuring Mr. Kaepernick in the 'Just Do It' campaign, Nike grossly insults the men and women who really do make sacrifices for the sake of our nation," the letter obtained by TMZ Sports reads.

"We are calling on all our member officers, their families and friends to join in boycotting all Nike products."

One of the largest police organizations in the nation is blasting Nike for the new Colin Kaepernick ad campaign ... saying Swoosh's definition of sacrifice is a slap in the face to those who risk their lives to protect the country.

The brand has been receiving both praise and criticism for Kaep's new "Just Do It" ad -- which features the words, "Believe in something. Even if it means sacrificing everything."

TMZ Sports has obtained a statement from the National Association of Police Organizations -- an interest group that represents more than 241,000 officers -- about the new campaign ... and they're PISSED.

"Oh, Nike! You are soooo brave!!" the statement says. "Holding up Colin Kaepernick as an example of someone who has 'sacrificed everything'. Right!"

"Come on over to Arlington National Cemetery, or to the National Law Enforcement Officers Memorial in Washington, D.C. or to the trauma unit of a military hospital if you want to learn the meaning of 'sacrifice' and why our flag means something to so many."

The sentiment has been echoed by people like John Rich and UFC star Tim Kennedy, among others.

On the flip side, star athletes from LeBron James to Serena Williams -- and even Tom Brady -- are giving Nike the thumbs-up.

And, Oakland Raiders owner Mark Davis told TMZ Sports he's "absolutely" supportive of the ad campaign.