Donald Trump is facing a backlash from sport stars after he criticised national football league (NFL) players who used the national anthem to protest.

The US president was speaking at a rally in Alabama where he told a cheering crowd: “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.’”

He later backed this up with a tweet saying: that if NFL players wanted “the privilege” of high salaries they “should stand for the National Anthem. If not, YOU’RE FIRED. Find something else to do!”

Former San Francisco 49ers quarterback Colin Kaepernick stirred a polarising national debate in 2016 after refusing to stand during pre-game renditions of the “Star Spangled Banner” to protest police violence against African-Americans.

Several players have made similar gestures of protest before games since Kaepernick initiated his protest.

If a player wants the privilege of making millions of dollars in the NFL,or other leagues, he or she should not be allowed to disrespect…. — Donald J. Trump ( realDonaldTrump) <a href="https://twitter.com/realDonaldTrump/status/911654184918880260">September 23, 2017</a></blockquote> <script async src="//platform.twitter.com/widgets.js" charset="utf-8"></script> <blockquote class="twitter-tweet" data-lang="en"><p lang="en" dir="ltr">...our Great American Flag (or Country) and should stand for the National Anthem. If not, YOU'RE FIRED. Find something else to do!</p>— Donald J. Trump ( realDonaldTrump) September 23, 2017

Responding to the attacks on football players, the NFL Commissioner Roger Goodell said on Saturday that Trump’s statements revealed an “unfortunate lack of respect” for the NFL and its players.

The players’ union said it would defend its players’ rights to freedom of expression.

NBA players joined in when Stephen Curry said he would not be a part of a planned visit to the White House.

Trump later withdrew the invitation in a tweet: “Going to the White House is considered a great honor for a championship team. Stephen Curry is hesitating, therefore invitation is withdrawn!”

Going to the White House is considered a great honor for a championship team.Stephen Curry is hesitating,therefore invitation is withdrawn! — Donald J. Trump (@realDonaldTrump) September 23, 2017

Curry said Trump’s response is not what leaders do: “It’s beneath the leader of a country to go down that route,” he said.

Lebron James offered his take on the issue calling Trump a “bum”.

While Kaepernick’s mother, not wanting to be left out of the debate, said: “Guess that makes me a proud bitch.”