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President Donald Trump was back on the attack on Monday, saying that "many" fans booed those players who took a knee during the weekend's NFL games.

"Many people booed the players who kneeled yesterday (which was a small percentage of total). These are fans who demand respect for our Flag!" Trump tweeted the morning after an NFL game day that saw players or owners on nearly all of the teams take action or issue statements against the president.

"The issue of kneeling has nothing to do with race. It is about respect for our Country, Flag and National Anthem. NFL must respect this!" Trump wrote, adding that he was "so proud of NASCAR and its supporters and fans."

"They won't put up with disrespecting our Country or our Flag — they said it loud and clear!"

Many people booed the players who kneeled yesterday (which was a small percentage of total). These are fans who demand respect for our Flag! — Donald J. Trump (@realDonaldTrump) September 25, 2017

The issue of kneeling has nothing to do with race. It is about respect for our Country, Flag and National Anthem. NFL must respect this! — Donald J. Trump (@realDonaldTrump) September 25, 2017

So proud of NASCAR and its supporters and fans. They won't put up with disrespecting our Country or our Flag - they said it loud and clear! — Donald J. Trump (@realDonaldTrump) September 25, 2017

But Trump got a prompt rebuke from Dale Earnhardt, Jr., one of the biggest names in NASCAR, who quoted President John F. Kennedy in his own tweet, writing that "All Americans R granted rights 2 peaceful protests."

"Those who make peaceful revolution impossible will make violent revolution inevitable-JFK," added Earnhardt.

All Americans R granted rights 2 peaceful protests

Those who make peaceful revolution impossible will make violent revolution inevitable-JFK — Dale Earnhardt Jr. (@DaleJr) September 25, 2017

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Several NASCAR team owners and drivers, including Richard Petty and Richard Childress, told the Associated Press Sunday that they wouldn't support anyone in the sport protesting during the national anthem.

But NASCAR itself spoke out Monday, issuing a statement that called "respect" for the national anthem "a hallmark" of pre-race events and but also emphasized "the right to peacefully express one’s opinion."

There were reports that fans of the New England Patriots fans and Buffalo Bills booed the players during their teams' anthem protests.

Patriots quarterback Tom Brady, who has called Trump a "friend," said Monday that he was opposed to the president’s tweets over the weekend.

"I certainly disagree with what he said and thought it was just divisive, and like I said, I just want to support my teammates," Brady said on Boston’s WEEI.

"I’m never one that says, 'Oh, that’s wrong or that’s right...' — but I do believe in what I believe in, and I believe in bringing people together, respect and love and trust," Brady added. "Those are the values my parents instilled in me." At the New England Patriots game against the Houston Texans, Brady linked arms with his teammates.

On Monday, White House Press Secretary Sarah Huckabee Sanders said Trump's tweets were "not about the president being against anyone" and that the commander-in-chief was merely "honoring our national anthem, honoring our national anthem" and " honoring the men and women who fought to defend it."

"That should certainly be a priority of the president," she said.

Sanders denied that Trump's tweets had anything to do with race, echoing what the president himself said on Sunday. "The president is not talking about race, the president is talking about pride in our country," she said.

Related: Trump Tweets Don’t Help TV Football Ratings

On Sunday, Baltimore Ravens coach John Harbaugh and Jacksonville Jaguars owner Shahid Khan were among those who joined players in protests. The Pittsburgh Steelers went a step further, announcing ahead of their matchup with the Chicago Bears that they would not leave their locker room for the national anthem.

The Seattle Seahawks players released a joint statement on Sunday explaining that they also would not participate in the national anthem.

"We will not stand for the injustice that has plagued people of color in this country," the statement said. "Out of love for our country and in honor of the sacrifices made on our behalf, we unite to oppose those that would deny our basic freedoms."

PHOTOS: NFL Players Lock Arms, Kneel During National Anthem to Protest Trump

The actions came after a fiery declaration from Trump on Friday night when he called those who kneel "sons of bitches" and tweeted that NFL players who don’t stand during the anthem should be "fired."

The NFL dismissed Trump's comments.

League spokesman Joe Lockhart said Monday that Trump's tweets prompted "incredibly thoughtful conversation" within the league "about very difficult issues."

"This is what real locker room talk is," Lockhart added, in a thinly veiled reference to words Trump himself used to describe the 2005 "Access Hollywood" tape that surfaced during the campaign that showed him making obscene comments about how he treated women.