Michael Moore has a blunt message for Republican presidential candidate Donald Trump: "We are all Muslim. Deal with it."

The documentary filmmaker and liberal activist wrote a letter to Trump late Wednesday, addressing the GOP front-runner's controversial call to ban all Muslims from entering the United States. The proposal, Moore said, was one made "in desperation and insanity."

"I was raised to believe that we are all each other's brother and sister, regardless of race, creed or color," Moore wrote in a Facebook message. "That means if you want to ban Muslims, you are first going to have to ban me. And everyone else."

Get Breaking News Delivered to Your Inbox

"We are all Muslim," he continued. "Just as we are all Mexican, we are all Catholic and Jewish and white and black and every shade in between... If you don't like living by these American rules, then you need to go to the time-out room in any one of your Towers, sit there, and think about what you've said."

Moore posted his letter to Facebook, along with a photo of himself standing outside Trump Towers with a "We are all Muslim" sign:

Today I went & stood in front of Trump Tower & held a sign until the police came. Then I went home & wrote Donald a... Posted by Michael Moore on Wednesday, December 16, 2015

In his letter, the filmmaker described his first meeting 17 years ago with the real estate mogul, in the green room of a talk show, where he was asked by a producer not to "go after" Trump, was, she told Moore "nervous" about being on the show with the famously confrontational documentary director.

"I was struck by how you, a self-described tough guy from Queens, seemed like such a fraidey-cat," Moore recalled, after speaking with Trump and noting how "relieved" the businessman looked when Moore politely introduced himself.

"You and I went on to do the show. Nothing untoward happened between us. I didn't pull on your hair, didn't put gum on your seat," he added. "'What a wuss,' was all I remember thinking as I left the set."

Moore ended his missive with a call for readers to sign a "We Are All Muslim" letter (found on his website) and then post a photo of themselves on social media sites with a sign and a similar hashtag declaration.

The filmmaker promised to post all the photos from Twitter, Facebook, and Instagram on his own website and then send them to Trump.

For his part, Trump has defended his proposal to bar Muslims from stepping on U.S. soil.

"We're talking about security. We're not talking about religion. We're talking about security," Trump said during Tuesday's fifth Republican debate. "Our country is out of control."

Moore has been similarly derisive of Donald Trump in the past, even calling him "bat-sh*t crazy" in an interview with Entertainment Weekly.

"See, I think people love bat-sh*t crazy," Moore said in September. "I think that that's just us loving entertainment. You know, Stephen Colbert did that character. He was playing bat-sh*t crazy Bill O'Reilly. This is actually Trump. This is not performance art."