Players are seeking to draw attention to what they say is racism by police
US President Donald Trump has claimed that his comments on NFL stars kneeling during the US national anthem are "nothing to do with race".
The president urged fans to stop going to games "until players stop disrespecting our Flag & Country".
He also tweeted that any players who do kneel should be fired or suspended.
If NFL fans refuse to go to games until players stop disrespecting our Flag & Country, you will see change take place fast. Fire or suspend!— Donald J. Trump (@realDonaldTrump) September 24, 2017
Players have been kneeling during The Star-Spangled Banner at the beginning of American Football games to draw attention to what they say is racism against African-Americans by police.
When asked to confirm whether he thought players who knelt should be fired he said: "It's very disrespect to our flag and to our country, so I certainly think the owners should do something about it."
When asked whether he thought his comments were inflaming racial tensions, Mr Trump added: "This has nothing to do with race. I never said anything about race.
"This has nothing to do with race or anything else. This has to do with respect for our country and respect for our flag."
Mr Trump's earlier comments resulted in a stream of protests from figures within the NFL and the acting community.
More than 150 players sat, knelt or raised their fists in defiance during the league's 13 games on Sunday - the biggest display of strength of feeling since the protests began in 2016.
Others stood but linked arms in an apparent show of solidarity.
Among those taking part were NFL players at London's Wembley Stadium during a guest-based match between the Baltimore Ravens and Jacksonville Jaguars.
Singer Stevie Wonder has also backed the growing movement, which has been christened #takeaknee.
In a tweet, Mr Trump said that "Standing with locked arms is good, kneeling is not acceptable. Bad Ratings!"
Great solidarity for our National Anthem and for our Country. Standing with locked arms is good, kneeling is not acceptable. Bad ratings!— Donald J. Trump (@realDonaldTrump) September 24, 2017
Avengers actor Chadwick Boseman was among those from the acting community who spoke out in solidarity, adding: "We're in a period of time where people have to decide what is patriotism.
"For the President of the United States to say that people can't... they can't display their version of patriotism. Because all that people are saying is... that America should live up to what its creed is. That's it.
"Until it does that, until black men are not brutalized in the street and treated unfairly, I'm going to acknowledge that it's not doing that. That is still an acknowledgement of the flag."
Actor John Leguizamo said: "#takingaknee today for people of color until we are all equal and treated fairly!"
The chairman of the Boston Patriots Robert Kraft also condemned the president's comments and backed the players.
He said: "I am deeply disappointed by the tone of the comments made by the President on Friday. I am proud to be associated with so many players who make such tremendous contributions in positively impacting our communities.
"Our players are intelligent, thoughtful and care deeply about our community and I support their right to peacefully affect social change."
The movement started more than a year ago when former San Francisco 49ers quarterback Collin Kaepernick refused to stand during the national anthem.