A wave of protests swept across the National Football League (NFL) on Sunday as United States (US) 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 take a knee through renditions of “The Star-Spangled Banner” as “sons-of-bit***s” who should be fired.

The US leader doubled down on those remarks in an early morning tweet, 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 and country, you will see change take place fast. Fire or suspend!” Trump wrote.

Yet players throughout America's most popular sport took a defiant stance just hours later, kneeling, linking arms or raising clenched fists during the anthem.

More than 150 players could be seen kneeling or sitting in the 14 games that took place on Sunday, easily the largest such demonstration since former San Francisco 49ers quarterback Colin Kaepernick first began protesting in 2016.

One of the biggest protests took place in the nation's capital, where almost the entire lineup of the Oakland Raiders team sat on their bench ahead of their game with the Washington Redskins.

A day of demonstrations began at the London game between the Jacksonville Jaguars and Baltimore Ravens at Wembley Stadium, where a large number of players from both teams knelt.

In Nashville, neither the Seattle Seahawks nor the Tennessee Titans took to the field to observe the national anthem.

“We will not stand for the injustice that has plagued people of this colour in this country,” Seattle players said in a statement just prior to kickoff.

'Deeply disappointed'

In Foxborough, around 15 members of the Super Bowl champion New England Patriots took a knee during the anthem.

Star quarterback Tom Brady stood but linked arms with his teammates.

Reports said the protests were greeted with scattered boos as some fans chanted “Stand up!”.

Patriots receiver Brandin Cooks, who scored the game-winning touchdown, later explained his decision to kneel, rejecting accusations it was unpatriotic.

“A lot of people think we're disrespecting the flag and the military, but my father was a marine, my uncle was a marine, [my] family fought in the Vietnam war; I have the utmost respect for the men and women that fight for our freedom,” Cooks told reporters.

In Chicago, the Pittsburgh Steelers chose to remain in their locker room during the anthem ahead of their clash with the Bears.

Buffalo Bills star Lesean McCoy, who stretched on the turf during the anthem, said, “I can't stand and support something where the leader of our country is acting like a jerk.”

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 responded to the protests on Twitter.

“Standing with locked arms is good, kneeling is not acceptable,” he wrote.

In subsequent remarks to reporters, Trump denied there was a racial dimension to his criticism of activist athletes, most of whom are black.

“This has nothing to do with race or anything else. This has to do with respect for our country and respect for our flag,” Trump said.

The players' 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 National Basketball Association (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 spread

Trump's outburst drew a stinging response from across the NBA, with Cleveland Cavaliers superstar LeBron James describing the US leader as a “bum.”

The trenchant response from NFL and NBA players to Trump's comments had shown signs of spreading to other leagues, with a baseball player kneeling during the anthem on Saturday.

In game one of the Women's National Basketball Association finals on Sunday, members of the Los Angeles Sparks remained in their dressing room during the anthem. Their opponents, the Minnesota Lynx, stood to attention with arms locked.

Track and field star Allyson Felix, a six-time Olympic gold medallist, also spoke out in support of the protests.

“Enough is enough. We have the power to create change,” she said.

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.

Critics counter that the protests are disrespectful of the country and its military.