Players from the Jacksonville Jaguars and the Baltimore Ravens knelt during the American anthem at Wembley on Sunday in defiance of United States president Donald Trump.
At a rally in Alabama, Trump had delivered a scathing attack on NFL players who opt to kneel in protest when the Star-Spangled Banner is played prior to matches.
He claimed team owners should sack any player involved in such a demonstration and that fans should leave the stadium if they see it.
The near packed-out Wembley contest between the Jags and Ravens offered the first chance for players to respond to those comments and over a dozen, from both the Ravens and Jaguars, took a knee as their national anthem played.
Those who did not kneel linked arms, including Jaguars owner Shad Khan, who stood between Marcedes Lewis and Telvin Smith on the sidelines.
Khan, who also owns Championship football club Fulham, has previously donated $1million to Trump's presidential inauguration.
When the game kicked off, over a dozen teams had made public statements in support of their players, but the Ravens and Jaguars had not.
Shortly after the contest got under way at Wembley, the Ravens posted a message on Twitter which read: "We recognise our players' influence. We respect their demonstration and support them 100 per cent. All voices need to be heard. That's democracy in its highest form."
Statement from Owner Steve Bisciotti. pic.twitter.com/bdKWJ4UpCy— Baltimore Ravens (@Ravens) September 24, 2017
The Jaguars tweeted a photo of Khan standing, arm in arm with Lewis and Smith, with the caption "Unity".
Trump had followed up Friday night's comment with a tweet the following day criticising NFL commissioner Roger Goodell, who had called the president's comments "divisive".
On Sunday Trump took to social media again prior to kick-off at Wembley.
"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!" he wrote from his account @RealDonaldTrump.
"...NFL attendance and ratings are WAY DOWN. Boring games yes, but many stay away because they love our country. League should back U.S."
Those who responded by taking a knee in London included high-profile stars like Terrell Suggs, Malik Jackson and Calais Campbell.
Further protests were expected back in the States across the later slate of games.
Green Bay Packers quarterback Aaron Rodgers, one of the league's most recognisable figures, put a photo on Instagram on Sunday afternoon of him kneeling alongside team-mates during a warm-up with the caption: "#unity #brotherhood #family #dedication #love".