The highly charged remarks, made during a rally in Alabama, led to criticism from the league’s commissioner and an outcry from players.
Protesting the national anthem is a fireable offense, President Trump argued on Friday, as he hammered the National Football League for being too soft on players who have knelt to denounce police brutality.
“Wouldn’t you love to see one of these N. F. L. owners, when somebody disrespects our flag, to say, ‘Get that son of a bitch off the field right now, he’s fired,’ ” Mr. Trump said during an appearance at a political rally in Alabama.
“When people like yourselves turn on television and you see those people taking the knee when they’re playing our great national anthem, the only thing you could do better is if you see it, even if it’s one player, leave the stadium,” he said. “I guarantee things will stop.”
Mr. Trump also criticized rule changes designed to protect players from injury and prevent concussions. “They’re ruining the game,” he said. “They’re ruining the game.”
The president’s comments prompted an outcry from players and a sharply worded statement from the N. F. L. commissioner, Roger Goodell, who called the remarks “divisive” and said they demonstrated a lack of respect for the league.
“The NFL and our players are at our best when we help create a sense of unity in our country and our culture,” Mr. Goodell said. “Divisive comments like these demonstrate an unfortunate lack of respect for the NFL, our great game and all of our players, and a failure to understand the overwhelming force for good our clubs and players represent in our communities.”
Mr. Trump’s remarks appeared to be directed at Colin Kaepernick, the former quarterback for the San Francisco 49ers who stopped standing for the national anthem last season, saying he was protesting violence against minorities by the police.
“I am not going to stand up to show pride in a flag for a country that oppresses black people and people of color,” Mr. Kaepernick said last year. “To me, this is bigger than football and it would be selfish on my part to look the other way. There are bodies in the street and people getting paid leave and getting away with murder.”
His actions inspired other players to do the same, and started a national debate about the intersection of activism and sports. Mr. Kaepernick opted out of his contract in March and is a free agent. Analysts have speculated that his advocacy is the reason he is not signed to a team.
Current and former N. F. L. players spoke out against Mr. Trump on Twitter and expressed their support for Mr. Kaepernick.
Mr. Trump’s comments came during a campaign rally in Alabama for Senator Luther Strange, who was appointed to the Senate this year and is facing Roy Moore in a Republican primary runoff.
By Saturday morning, the president appeared to have another league on his mind. He tweeted that the National Basketball Association player Stephen Curry, a two-time M. V. P., was not welcome at the White House.
The team that wins the N. B. A. championship is customarily invited to visit. Mr. Curry’s team, the Golden State Warriors, won this year, but he said on Friday that he did not want to go.
