The US National Football League is embroiled in a standoff with President Donald Trump after it said players would be allowed to take the knee" during the American national anthem in protest against racism. After the NFL announced its U-turn, Trump tweeted late on Friday night: "We should be standing up straight and tall, ideally with a salute, or a hand on heart. There are other things you can protest, but not our Great American Flag - NO KNEELING!" The NFL commissioner Roger Goodell said ... that the league's earlier ban on players taking the knee had been mistaken. "We were wrong for not listening to NFL players earlier, and encourage all to speak out and peacefully protest," Goodell said. The change in the NFL's position came after some players urged the league to "condemn racism and the systemic oppression of black people". The practice of taking the knee during the national anthem before games started in 2016 as a protest by black player Colin Kaepernick against racial injustice. In the past two weeks, it has become an international symbol of opposition to racism. Trump has frequently denounced the action. Two years ago, he praised the NFL's ban on taking the knee during the pre-game national anthem, saying: "You have to stand proudly for the national anthem. Or you shouldn't be playing, you shouldn't be there. Maybe you shouldn't be in the country." Last week, the president criticized New Orleans Saints quarterback Drew Brees for dropping his opposition to NFL kneeling protests.