Across the United States, players in the National Football League are sitting out, kneeling or linking arms during the pre-game national anthems.
Even at Wembley Stadium in London, NFL teams playing a game there locked arms in unity.
It came hours after US president Donald Trump called on fans to boycott teams that do not discipline players who protest.
The gesture, known as the “Take A Knee” protest, started last year when the San Francisco 49ers’ Colin Kaepernick began kneeling during the national anthem instead of standing.
He said at the time it was to protest racial injustice and police brutality against black and minority groups.
But Donald Trump says that is an insult to America and Americans.
“Wouldn’t you love to see one of these NFL owners, when somebody disrespects our flag, to say, ‘Get that son of a (bleep) off the field right now, out, he’s fired. He’s fired!'”
Addressing a crowd in the southern state of Alabama, Mr Trump has described the gesture as total disrespect for the country’s heritage and offered a recommendation for fans.
“If you see it, even if it’s one player, leave the stadium. I guarantee things will stop.”
President Trump has since refused to back down, saying all professional athletes who disrespect the flag should be fired.
On his favourite medium, Twitter, he preemptively withdrew a White House invitation to basketball superstar Stephen Curry.
That prompted a social-media firestorm.
Fellow basketball star LeBron James, who has nearly as many Twitter followers as Mr Trump, tweeted, “You bum, going to White House was a great honour until you showed up.”
He later issued a video saying Americans need to come together at this critical time.
“For him to use this platform to divide us even more is not something I can stand for and is not something I can be quiet about. You look at him kind of asking the NFL owners to get rid of players off the field because they’re exercising their rights, and that’s not right. And then, you know, when I wake up, I see that a colleague of mine has been uninvited to something that he said he didn’t even want to go to in the first place, to the White House, that’s just something I can’t stand for, man. We’ve got Jamele Hill, and Colin Kaepernick, and all these people are speaking up, and it’s for the greater cause.”
The NFL players union and many players themselves have defended what they see as free speech.
The owner of the New England Patriots NFL team, Robert Kraft, supported Donald Trump in his presidential election campaign.
But he has issued a statement saying he is deeply disappointed by the President’s comments.
“I am proud to be associated with so many players who make such tremendous contributions in positively impacting our communities. Our players are thoughtful, intelligent and care deeply about our community. I support their right to peacefully affect social change and raise awareness in a manner that they feel is most impactful.”
Stars from other sports, including baseball, have now joined in, and even soul-music legend Stevie Wonder has lent his support at the start of one of his concerts.
Donald Trump’s treasury secretary, Steve Mnuchin, says the NFL players can practice free speech “in their own time.”
“The NFL has all different types of rules. You can’t have stickers on your helmet. You have to have your uniforms tucked in. What the President is saying — and I think the owners should meet and — they should vote on a rule. This is about respect for our military. This is about respect for our first responders. This is not about Republicans or Democrats. Players have the right for free speech off the field.”
But US House Democratic leader Nancy Pelosi says President Trump would be ill-advised to continue the battle.
“Somebody like Colin Kaepernick doing what he did says, ‘This flag enables me to do this. This national anthem enables me to do this.’ This is about freedom of expression. Instead of resorting to what he is doing now … it’s unfortunate. I think he’s ill-advised, if advised at all, to go down this path.”