Baltimore Ravens kneel at Wembley Stadium during the US national anthem
Baltimore Ravens and Jacksonville Jaguars players protested during the US national anthem before Sunday's NFL match at Wembley Stadium.
Several went down on one knee in defiance of US President Donald Trump's suggestion that those who protest during the anthem should be fired.
Last year, quarterback Colin Kaepernick knelt for the anthem in protest against racial injustice and police brutality.
Further protests are expected during the other NFL games on Sunday.
At Wembley more than 20 players and staff from both sides either knelt or linked arms during the anthem.
Jaguars owner Shad Khan, who donated $1m to Trump's inauguration committee, also linked arms with two of his players.
"I met with our team captains prior to the game to express my support for them, all NFL players and the league following the divisive and contentious remarks made by President Trump, and was honoured to be arm in arm with them, their teammates and our coaches during our anthem," Khan said.
"Our team and the NFL reflects our nation, with diversity coming in many forms - race, faith, our views and our goals.We have a lot of work to do, and we can do it, but the comments by the president make it harder."
Ravens owner Steve Bisciotti added:"We recognise out 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."
The players all stood when God Save the Queen was played after the US national anthem.
Trump told a Republican rally in Alabama on Friday that the protests showed "disrespect of our heritage".
He then followed that up with further criticism on Twitter, writing on Sunday:"If NFL fans refuse to go to games until players stop disrespecting our flag and country, you will see change take place fast.Fire or suspend!"
NBA stars have also become involved in the issue, with LeBron James describing US President Donald Trump on Saturday as a "bum" over comments he made about fellow basketball star Steph Curry.
Trump said the Golden State Warriors were no longer invited to the White House after their superstar Curry, 29, said he did not want to attend "to show that we won't stand for the things (the president) has said".
"Going to White House was an honour until you showed up," James, 32, said.
What is the anthem protest?
Following 29-year-old Kaepernick's refusal to stand for the anthem more players have since joined in by taking a knee or raising a fist during the anthem.
Speaking on Friday, Trump said:"Wouldn't you love to see one of these NFL owners, when somebody disrespects our flag, to say, 'Get that son of a bitch off the field right now, out, he's fired.He's fired'," the former host of The Apprentice said.
"You know, some owner is going to do that.He's going to say, 'That guy that disrespects our flag, he's fired'.And that owner, they don't know it [but] they'll be the most popular person in this country."
Having opted out of his contract with the San Francisco 49ers in the off-season, Kaepernick - who began his protests because he wanted to start a nationwide debate - remains a free agent.
National Football League commissioner Roger Goodell supported his players by explaining they had raised millions of dollars for recent disaster relief efforts and were involved in community programmes.
"There is no better example than the amazing response from our clubs and players to the terrible natural disasters we've experienced over the last month," he said.
"Divisive comments like these demonstrate an unfortunate lack of respect for the NFL, our great game and all of our players, and a failure to understand the overwhelming force for good our clubs and players represent in our communities."
The NFL Players' Association president Eric Winston said Mr Trump's comments were "a slap in the face to the civil rights heroes of the past and present".
In other reaction:...
- New England Patriots CEO Robert Kraft said he was "deeply disappointed" by the comments, and that he supported players' rights to protest
- Miami Dolphins owner and founder Stephen Ross said the US needed "unifying leadership right now, not more divisiveness"
- Jed York, CEO of Kaepernick's former team the San Francisco 49ers said he would continue to support his players, calling the comments "callous and offensive"
Jacksonville Jaguars owner Shahid Khan (centre) - who also owns Championship football club Fulham - displayed his unity with his players Some Jacksonville Jaguars (left) kneel during the US national anthem in defiance of president Donald Trump's suggestion