Players from the Baltimore Ravens and Jacksonville Jaguars knelt during the playing of the US national anthem in London on Sunday, in the first NFL game since President Donald Trump condemned players who are protesting against racial inequality.
Several players from both teams took a knee during the rendition of The Star-Spangled Banner as it was sang at Wembley Stadium while other players stood and locked arms in solidarity.
It was one of the largest protests by NFL players during the national anthem since former San Francisco 49ers quarterback Colin Kaepernick first began refusing to stand for the anthem in 2016.
The league is expected to see widespread protests later Sunday when the full schedule of games take place across the US.
Sunday's protest came after Trump triggered an uproar on Friday by saying that players who protested during the anthem should be fired, branding them "sons of bitches" at a rally in Alabama.
Trump's remarks drew widespread criticism from across the National Football League, with players, owners and league chiefs lining up to condemn his tirade.
An unrepentant Trump doubled down on his comments early on Sunday however, repeating his demand for players to be fired and urging fans to boycott games if 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.
"...NFL attendance and ratings are WAY DOWN. Boring games yes, but many stay away because they love our country. League should back US."
On Saturday, NFL Commissioner Roger Goodell issued a statement criticizing Trump's comments.
"Divisive comments like these demonstrate an unfortunate lack of respect for the NFL, our great game and all of our players," Goodell wrote.
The protests began in August 2016, when Kaepernick refused to stand during playing of the national anthem in protest at the treatment of minorities by law enforcement.
It followed the high-profile deaths of several unarmed black men during confrontations with police officers.
Kaepernick was widely ostracised for his stance and has since been unable to find an employer in the NFL, which is America's most popular professional sports league.
However support for Kaepernick's position has since snowballed, given added momentum by Trump's latest salvo.