Mr Trump attacked dissenting players at a rally in Alabama
At a rally on Friday night, Mr Trump said NFL players who protested during the playing of the national anthem should be fired by their team - referencing a controversial string of protests over race relations started by player Colin Kaepernick last year.
"Wouldn't you love to see one of these NFL owners, when somebody disrespects our flag, to say, 'Get that son of a bitch off the field right now...he is fired," he told a cheering crowd.
How did the NFL respond?
NFL commissioner Roger Goodell released a statement saying "divisive comments like these demonstrate an unfortunate lack of respect".
President Trump, however, doubled down on his comments, tweeting that the commissioner was "trying to justify the total disrespect certain players show to our country.Tell them to stand!"
The NFL Players' Association, meanwhile, said the president had crossed the line by effectively telling players to just "shut up and play".
Association president Eric Winston said Mr Trump's comments were "a slap in the face to the civil rights heroes of the past and present" and those who spilled blood to uphold American values.
Who else has joined in the criticism?
Jed York, CEO of the San Francisco 49ers - the former team of Colin Kaepernick - said he would continue to support his players' "pursuit of positive change".
"The callous and offensive comments made by the President are contradictory to what this great country stands for," he said in a statement.
He said players were exercising their rights "in order to spark conversation and action to address social injustice."
There has been no comment from many teams, including New York Jets owner Woody Johnson - a wealthy businessman and Trump campaign donor who was appointed as Trump's ambassador to the UK.
Colin Kaepernick's mother, Teresa, also joined in the backlash, referencing Trump's "son of a bitch" comment and tweeting:"Guess that makes me a proud bitch!"
On Saturday, Mr Trump withdrew an invitation to the White House from basketball champions the Golden State Warriors after one player, Stephen Curry, said he did not want to attend.
Curry - NBA's most valuable player in 2015 - said he wanted to show that he and other players did not stand for "the things that he's said and the things that he hasn't said in the right times".
"Going to the White House is considered a great honor for a championship team," Mr Trump tweeted afterwards."Stephen Curry is hesitating, therefore invitation is withdrawn!"
In response, triple NBA champion LeBron James, one of the sport's foremost stars, labelled the President a "bum".
"Going to White House was an honour until you showed up," he said.
Retired star Kobe Bryant said Mr Trump was a president "whose name alone creates division and anger" and could not possibly achieve his campaign slogan to "make America great again".
The Golden State Warriors, meanwhile, said the team had clearly understood "that we are not invited" to the White House, but would visit Washington DC on its own "to celebrate equality, diversity, and inclusion".
NBA Commissioner Adam Silver said he was disappointed the team would not be visiting the White House, but was "proud" of the players for speaking out.
Analysis:Trump knows his audience
Anthony Zurcher, North America reporter, Alabama
Donald Trump spoke for nearly an hour and a half at his rally in Alabama on Friday.The audience of 10,000 was enthusiastic, but one of his biggest applause lines - outside of swipes at North Korea and Hillary Clinton - was when the president took aim at any "son of a bitch" in the NFL who protests during the national anthem.
The crowd went wild - and the president kept going, lamenting recent rule changes, in response to evidence of devastating brain injuries to players, that seek to punish the most ferocious hits in the game.
On Saturday morning, like an accomplished comic returning to material that tested well at a local comedy club, the president took to Twitter to fire off line after line attacking the NFL - and athletes in any sport who may have taken issue with his earlier comments.
This is a fight Mr Trump relishes.He knows his base will flock to him when he questions the patriotism of wealthy athletes.Lines about tax cuts and healthcare reform on Friday night were duds.No one cared much when he spoke of striking deals with Democrats.
The president knows his audience.And he knows how to change the subject....