Players in the National Football League (NFL) in the US have knelt and locked arms in a wave of protests at President Donald Trump's "offensive" and "divisive" comments aimed at their teammates.
In a growing row with the sports world, Mr Trump insisted that players who failed to stand during the national anthem should be fired or suspended.
Sunday's protests are the largest of their kind since they began last year.
Players have been protesting at racial injustice and police violence.
New England Patriots quarterback Tom Brady joined his teammates in Foxborough, Massachusetts, as they locked arms while several others decided to kneel before facing the Houston Texans.
The Pittsburgh Steelers, meanwhile, failed to show at all during the national anthem ahead of their game in Chicago against the Chicago Bears, who stood on the sidelines with their arms locked.
Steelers coach Mike Tomlin said his team would be "unified in their actions", adding that the decision to remain in the locker rooms was not "disrespectful", the New York Times reports.
Separately, Philadelphia Eagles fans clashed with protesters ahead of a game in their home city against the New York Giants over whether NFL players should be expressing their political views on the field during national anthem.
Earlier, at the start of the first game since Mr Trump's remarks, many players and staff from the Jacksonville Jaguars and the Baltimore Ravens knelt in London.
Jacksonville Jaguars owner Shahid Khan - who donated $US1m to the Trump campaign - locked his arms with players in an unusual scene, as owners rarely join players on the pitch.
Mr Khan later issued a statement condemning Mr Trump's recent comments as "divisive and contentious".
In two tweets early on Sunday just hours before a series of NFL games, Mr Trump repeated his call for clubs to punish players who protested during the US anthem.
He was referring to a string of controversial protests started by player Colin Kaepernick last year when he sat or kneeled during the anthem to highlight the treatment of black Americans.
To a crowd of cheering supporters on Friday, Mr Trump asked: "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 ... he is fired'?"
Mr Trump's remarks on Friday have been widely criticised, with NFL commissioner Roger Goodell saying in a statement that "divisive comments like these demonstrate an unfortunate lack of respect".
President Trump, however, doubled down on his comments in a tweet, saying: "Tell them to stand!".
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".
On Saturday night, the Oakland Athletics' Bruce Maxwell became the first Major League Baseball player to kneel in protest during the anthem.
His father is in the military and he was born on an army base, US media report. He told a reporter he was "kneeling for people that don't have a voice".
Mr Trump is also facing criticism after withdrawing an invitation to the White House to basketball champions the Golden State Warriors after one player, Stephen Curry, said he did not want to attend.
Curry - NBA's top performer in 2015 - said he wanted to show that he and other players did not stand for "the things that he's said and the things that he hasn't said in the right times".
"Going to the White House is considered a great honor for a championship team," Mr Trump tweeted afterwards. "Stephen Curry is hesitating, therefore invitation is withdrawn!"
The Golden State Warriors said the team had clearly understood "that we are not invited" to the White House but would visit Washington DC on its own "to celebrate equality, diversity, and inclusion".