US President Donald Trump has said his comments about national anthem protests have "nothing to do with race".
A number of sports players and teams demonstrated during the US national anthem over the weekend.
The protests involved players kneeling, linking arms or staying in the locker room during the Star-Spangled Banner.
The president first said on Friday at a rally in Alabama that players who fail to stand during the national anthem should be fired or suspended.
His criticism seemed to galvanise players, teams and the league to assert what they saw as a right to freely express political convictions.
"The issue of kneeling has nothing to do with race," Mr Trump tweeted on Monday morning, reiterating his statement to reporters a day earlier.
"It is about respect for our Country, Flag and National Anthem. NFL must respect this!"
Recording artists Stevie Wonder, John Legend and Pharrell Williams have shown solidarity by joining in the demonstrations at weekend concerts.
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
The president's latest foray into the US culture wars comes after he was widely criticised for appearing to say anti-racism demonstrators were just as bad as far-right activists, after clashes at a deadly white supremacist rally in Virginia in August.
Why did the protests start?
National Football League (NFL) player Colin Kaepernick first sat down during the anthem in preseason in 2016.
"I am not going to stand up to show pride in a flag for a country that oppresses black people and people of colour," he said.
"To me, this is bigger than football and it would be selfish on my part to look the other way."
Kaepernick continued to demonstrate amid fierce criticism but this season remains a free agent.
Some commentators suggest he may have been "blackballed" from the sport as clubs fear a backlash for signing him.
What did Trump say?
The US president waded into the argument on Friday when he asked a crowd of supporters: "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 has also tweeted multiple times on the issue, suggesting the NFL should change its laws to stop players demonstrating.
But sports players responded with widespread protest action during the weekend's sports games.
Are players allowed to do it?
A US law called the Flag Code covers the etiquette around the National Anthem.
It says persons present around the national anthem are expected to stand with their hand on their heart and face a flag if there is one present.
However, the code is never enforced and there is no official punishment for breaching it.
In any case, players' flouting of tradition has angered many sports fans.
Who joined in protests?
Some national anthem singers and high-profile artists have joined in by falling to their knees.
During Sunday's NFL games:
- Neither the Seattle Seahawks nor the Tennessee Titans turned out for the national anthem before kick-off at their game, hours after the Pittsburgh Steelers did the same in Chicago (except Alejandro Villanueva, a veteran who served in Afghanistan)
- The Chicago Bears stood on the sidelines with their arms locked, as did New England Patriots star quarterback Tom Brady and teammates at another game. Some Green Bay Packers and Cincinnati Bengals players also linked arms
- The anthem singer at the Seahawks-Titans game kneeled at the end of the performance, as did singer at the Lions-Falcons game, who also raised his fist
- Philadelphia Eagles fans clashed with protesters ahead of a game in their home city against the New York Giants
- The NFL itself has criticised Mr Trump's remarks, with commissioner Roger Goodell saying "divisive comments like these demonstrate an unfortunate lack of respect"
What about Trump's NFL allies?
Three NFL owners who donated $1m apiece to the president's inauguration either joined the protests or criticised him.
Jacksonville Jaguars owner Shahid Khan locked arms with his players, in an unusual scene, as owners rarely join players on the pitch.
Washington Redskins owner Daniel Snyder also linked arms with cornerbacks as the national anthem played before Sunday night's game.
New England Patriots owner Robert Kraft, a personal friend of Mr Trump, said he was "deeply disappointed" by his "tone".
Meanwhile Patriots quarterback Tom Brady, a golf buddy of Mr Trump, told a Boston radio show on Monday the president's comments were "just divisive".
Have protests spread to other sports?
On Saturday night, the Oakland Athletics' Bruce Maxwell became the first Major League Baseball player to kneel in protest during the anthem.
NBA legend Michael Jordan, owner of the Charlotte Hornets, also weighed in, telling the Charlotte Observer newspaper: "Those who exercise the right to peacefully express themselves should not be demonized or ostracized."
Mr Trump is also facing criticism for withdrawing a White House invitation 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".
However, the Pittsburgh Penguins hockey team confirmed they would attend the White House, despite the controversy.
NBA superstar LeBron James of the Cleveland Cavaliers said anthem kneeling is not disrespectful and anyone who suggests otherwise is trying to divide.
A number of Nascar bosses have come out and said they will not tolerate any kind of demonstrations in their sport.