US President Donald Trump is facing growing condemnation from the sports world after his criticism of players.
On Friday, Mr Trump said the National Football League (NFL) should fire players who protest during the US anthem.
Top football players have attacked Mr Trump in response. The sport's boss called his comments "divisive".
Hours before Sunday's games, Mr Trump urged fans to boycott games as a way of making change "take place fast".
Warning: This article contains language some readers may find offensive.
What did Mr Trump say?
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.
He was referring to a controversial string of protests started by player Colin Kaepernick last year when he sat or kneeled during the anthem to highlight the treatment of black Americans.
"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.
Kaepernick led the San Francisco 49ers to the 2013 Super Bowl, but his status in the team has declined since, and he lost his starting place last year.
No team has offered him a job as a quarterback - and his supporters say he is being pushed out because of his political action.
But others say his status as a "free agent" is down to his performance on the field.
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 in a tweet.
The NFL Players' Association said the president had crossed a 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".
Who else has joined in the criticism?
On Saturday night, the Oakland Athletics' Bruce Maxwell became the first Major League Baseball player to kneel in protest during the national anthem, mimicking the gesture of protest started by Colin Kaepernick.
His father is in the military and he was born on an army base, US media report. He told a reporter he was "kneeling for people that don't have a voice".
Jed York, CEO of the San Francisco 49ers - Kaepernick's former team - said he would continue to support his players.
"The callous and offensive comments made by the President are contradictory to what this great country stands for," he said in a statement.
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 his ambassador to the UK.
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 to basketball champions the Golden State Warriors after one player, Stephen Curry, said he did not want to attend.
Curry - NBA's top performer 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 also tweeted his support.
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.
Separately, 2017 college basketball champions the North Carolina Tar Heels announced they, too, would not be going to the White House to celebrate their victory, despite being invited.
A spokesman said a suitable date that worked for both parties could not be found.
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 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.