Washington - A wave of protests swept across the National Football League on Sunday as President Donald Trump escalated his feud with players who kneel during the US national anthem to draw attention to racial injustice.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.Yet players throughout America's most popular sport took a defiant stance on Sunday, in the largest such demonstration since former San Francisco 49ers quarterback Colin Kaepernick started the protests in 2016.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.Others stood with their heads bowed and arms interlocked, among them Jacksonville owner Shad Khan - who donated $1m to Trump's election campaign in 2016.Khan later issued a statement slamming Trump's comments as "divisive and contentious."'Deeply disappointed'In Foxborough, around 15 members of the Super Bowl champion New England Patriots kneeled during the anthem.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.""These are very divisive times for our country," Tomlin told CBS television.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.Sunday's protests were the latest twist in a bitter war of words between Trump and US professional sports.On Saturday he had also drawn a furious backlash from NBA stars after stating on Twitter that the champion Golden State Warriors and star Stephen Curry would not be invited to attend a White House reception.Protests spreadCurry had on Friday indicated he would not attend the White House reception if invited."Going to the White House is considered a great honor for a championship team. Stephen Curry is hesitating, therefore invitation is withdrawn!" Trump wrote.Trump's outburst drew a stinging response from across the NBA, with Cleveland Cavaliers superstar LeBron James describing the US leader as a "bum"."Going to White House was a great honor until you showed up," James wrote.The trenchant response from NFL and NBA players to Trump's comments had shown signs of spreading to other sports.On Saturday, Major League Baseball witnessed its first protest: a black Oakland Athletics catcher named Bruce Maxwell knelt during the anthem before a game in California. Track and field star Allyson Felix, a six-time Olympic gold medallist, also spoke out in support of the protests."Grateful to all who are using their voice during this pivotal moment," Felix wrote on Twitter. "Enough is enough. We have the power to create change."Kaepernick's protest was aimed at drawing more attention to treatment of minorities in America following a spate of deadly police shootings of black men. He has since been unable to sign with any NFL team, and some say he was blackballed. But support for his position snowballed and other players have started protesting in the same way he did.Defenders of players such as Kaepernick say they have every right to express their social views at game time. Critics counter that fans who pay big money to attend NFL games or watch them on TV should not have to put up with political statements by players, and that the protests are disrespectful of the country and its military.