Trump ignited a firestorm of criticism after comments on Friday in which he described NFL players who chose to kneel through renditions of The Stars and Stripes as "sons of bitches" who should be fired.
The US leader doubled down on those remarks early Sunday, urging fans to boycott the NFL as long as the protests continued.
"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!" Trump wrote on Twitter.
The first mass protest took place at the NFL's London game between the Jacksonville Jaguars and Baltimore Ravens at Wembley Stadium, where a large number of players from both teams knelt.
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Others stood with their heads bowed and arms interlocked, among them Jacksonville owner Shad Khan -- who donated $1 million to Trump's election campaign in 2016.
Khan later issued a statement slamming Trump's comments as "divisive and contentious."
Star quarterback Tom Brady stood but linked arms with his team-mates. Reports said the protests were greeted with scattered boos as some fans chanted "Stand up!"
Patriots owner Robert Kraft, a friend of Trump who also donated to his campaign, issued a statement saying he was "deeply disappointed" by Trump's remarks on Friday.
In Chicago, the Pittsburgh Steelers chose to remain in their locker room during the anthem ahead of their clash with the Bears.
Steelers coach Mike Tomlin, who is black, said the decision was not intended to be disrespectful but rather calculated to "remove ourselves from the circumstance."
In Detroit, meanwhile, the singer of the national anthem Rico LaVelle dramatically dropped to his knee at the end of his rendition. At least eight Detroit Lions players were seen kneeling during the anthem while others linked arms.
Trump later responded to the protests on Twitter. "Great solidarity for our National Anthem and for our Country. Standing with locked arms is good, kneeling is not acceptable," he wrote.
Yesterday's protests were the latest twist in a bitter war of words between Trump and US professional sports.
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