The gesture of kneeling in protest during the traditional playing of “The Star Spangled Banner” was adopted by some black American football players over the last year to protest against racial disparities in the criminal justice system.
On Saturday, the US President tweeted: “Very important that NFL players stand tomorrow, and always, for the playing of our National Anthem.
“Respect our flag and our country!”
Later President Trump tweeted embedded video of fans and players standing at an NHL game, writing “19,000 respecting our National Anthem!”
However, other Twitter users posted that the video Mr Trump put up was nearly a year old.
On Tuesday, Mr Trump called on the league to ban players from kneeling in protest at games while the national anthem is played, tweeting: “The NFL has all sorts of rules and regulations. The only way out for them is to set a rule that you can’t kneel during our National Anthem!”
And on Thursday he continued the crusade, drawing a rebuke from the NFL after he said football team owners are afraid of their players.
The Republican President in an interview said that he is friends with many NFL team owners and they were “in a box” over how to handle the kneeling protests of racial disparities in the country.
He added: ”They say, ‘We are in a situation where we have to do something.’ I think they’re afraid of their players, you want to know the truth. And I think it’s disgraceful.”
The NFL rejected the president’s remarks as not factual.
Mr Trump also called, again via Twitter, for fans to boycott NFL games.
A week ago coaches, support staff and even some owners joined team members in silent support by kneeling, linking arms or staying off the field during the anthem.
The NFL national anthem protest was started by quarterback Colin Kaepernick, who began sitting or kneeling during the anthem last year in protest against police brutality and the treatment of black Americans.
Mr Kaepernick is currently a free agent after leaving the San Francisco 49ers in March 2017.
There are suggestions he has struggled to find a club because of his involvement with the politically sensitive NFL national anthem protest.