At a political rally in Alabama on Friday, President Trump had clear advice for N.F.L. owners if one of their players followed the lead of Colin Kaepernick, the former Super Bowl quarterback who last season began kneeling during the national anthem to protest police injustice against minorities. “Get that son of a bitch off the field right now,” he suggested.

The reaction was swift and stunning in its breadth — even as Mr. Trump went further on Sunday by calling for fans to boycott games unless the league fires or suspends players who refuse to stand for the national anthem.

More players than ever took a knee and many teams locked arms in solidarity. Owner after owner slammed Mr. Trump. Robert Kraft, the New England Patriots owner who is reportedly among several owners who donated $1 million to Trump’s inaugural celebrations, said he was “deeply disappointed” by Mr. Trump’s comments and defended his players’ right to “peacefully affect social change and raise awareness in a manner they feel is most impactful.”

The former N.F.L. coach Rex Ryan, who introduced Mr. Trump at a rally during the campaign, was furious and bitter at now being tied to Mr. Trump. “I never signed up for that,” Mr. Ryan said on ESPN’s “Sunday N.F.L. Countdown,” where he is a commentator.