But the acts of defiance received a far more mixed reception from fans, both in the stadiums and on social media, suggesting that what were promoted as acts of unity might have exacerbated a divide and dragged yet another of the country’s institutions into the turbulent cross currents of race and politics.
At Lincoln Financial Field in Philadelphia, videos posted on social media showed some Eagles fans yelling at anti-Trump protesters holding placards. At MetLife Stadium in East Rutherford, N.J., before the Jets played the Dolphins, many fans, a majority of them white, said they did not support the anthem protests but also did not agree with the president’s view that players should be fired because of them.
“I’m a Republican who voted for him, but I think this is a battle he doesn’t need to get into,” said Greg Zaccaria, 61, from White Plains, who said he had been a Jets season-ticket holder since 1978. Yet he objected to the anthem protests, saying, “I understand what they’re trying to get at, I just think there are better ways of expressing yourself.”
Mr. Trump, in a speech on Friday and a weekend-long series of tweets, had all but baited athletes and the league to respond by saying that those who do not stand for the anthem should be fired. He added that the league was in decline for tolerating the protests and for taking steps to reduce brain damage among players.
As the sideline demonstrations unfolded on Sunday, Mr. Trump wrote on Twitter: “Great solidarity for our national anthem and for our Country. Standing with locked arms is good, kneeling is not acceptable. Bad ratings!”
And before boarding Air Force One in the evening, Mr. Trump told reporters that his comments had “nothing to do with race or anything else — this has to do with respect for our country and respect for our flag.”
Still, players and team officials said they had made the gestures, including locking arms, in solidarity with players who had demonstrated during the anthem, not to support Mr. Trump.
There was a variety of protests on the sidelines Sunday. All but one player from the Pittsburgh Steelers, one of the league’s most celebrated teams, refused to go out for the anthem. The lone exception was Alejandro Villanueva, a veteran of the war in Afghanistan.
The Steelers, who were playing the Bears, were booed heavily by fans in Chicago when they ran onto the field after the anthem.
The Steelers, along with the Tennessee Titans and the Seattle Seahawks, who were playing each other and similarly skipped the anthem, broke a league rule requiring athletes to be present for the anthem, though a league executive said they would not be penalized.
“We will not stand for the injustice that has plagued people of color in this country,” Seahawks players said in a statement before the game. “Out of love for our country and in honor of the sacrifices made on our behalf, we unite to oppose those that would deny our most basic freedoms.”
Even stars who normally shy from controversy took a stand.
Green Bay Packers quarterback Aaron Rodgers posted a photograph of himself kneeling with three of his teammates during warm-ups before the game, and New England Patriots quarterback Tom Brady, a supporter of the president, “liked” Rodgers’s photo. Later, before the Patriots’ game against the Houston Texans in Foxborough, Mass., Brady locked arms with a teammate on the field and placed his hand over his heart during the anthem. Rodgers stood during the anthem, but three of his teammates sat on the bench.
A dozen or so Patriots knelt during the anthem, prompting some fans at Gillette Stadium to boo.
Still, one of the more surprising reactions came from the Patriots’ owner, Robert K. Kraft, a friend and campaign donor of the president who has invited Mr. Trump to sit with him at home games, as well as from other owners who were considered bedrock supporters of the president.
“I am deeply disappointed by the tone of the comments made by the President on Friday,” Kraft said in a statement hours before the New England game, adding that he supported players’ “right to peacefully affect social change and raise awareness in a manner that they feel is most impactful.”
E. Stanley Kroenke, the owner of the Los Angeles Rams, and Martha Firestone Ford, the owner of the Detroit Lions, both of whom lean to the right politically, also scolded the president.
Ford said “negative and disrespectful comments” were “contrary to the founding principles of our country, and we do not support those comments or opinions.”
She and the Atlanta Falcons’ owner, Arthur M. Blank, who donates to many Democratic causes, linked arms with players during the anthem before the Falcons-Lions game in Detroit. The singer Rico LaVelle went down on one knee as he finished singing the anthem.
The demonstrations intensified what was already a divisive debate that began last season when Colin Kaepernick, then a quarterback for the San Francisco 49ers, began kneeling during the playing of the national anthem, to highlight, he has said, police brutality and racial injustice.
He left the team after last season and has not played in the league since, inspiring questions over whether teams are punishing him, while many other players have knelt or made gestures in support of him or other social causes during the anthem.
Kaepernick has not commented, and his social media accounts were largely quiet on the president and the new round of protests.
But the fallout from Mr. Trump’s remarks spilled over into other sports.
In a tweet Friday, Mr. Trump disinvited the Golden State Warriors, the N.B.A. champions, to any traditional White House visit, after members of the team, including its biggest star, Stephen Curry, were critical of him. But on Sunday, the N.H.L. champion Pittsburgh Penguins said they would go to the White House, and declared such visits to be free of politics.
To promote the idea of the N.F.L. as a unifying force, the league was planning to replay a television ad called “Inside These Lines” on NBC on Sunday night when the Oakland Raiders played the Washington Redskins outside the nation’s capital.
While none of the Jets’ players knelt during the playing of the anthem, they locked arms on the sideline and were joined by some of the team’s administrators. On the other side of the field, four Miami Dolphins players — Maurice Smith, Kenny Stills, Thomas and Laremy Tunsil — knelt, while the team’s owner, Stephen M. Ross, locked arms with two of his players.
Two years ago, Mr. Ross started a nonprofit organization to combat racism and discrimination.
Outside the stadium in East Rutherford, Julie and Vin Santomero, who brought their sons to the game, said they also did not want to see protests at a sporting event because they attended games to get away from politics. “It’s a football game,” Mrs. Santomero said. “They’re here to play the game.”
Jesse Melendez, 29, of Dix Hills, N.Y.; Je’anna Pulistar, 29, of Lindenhurst, N.Y.; Roger Guevara, 29, of Yonkers; and Genesis Pineda, 27, of Yonkers, took the opposing view by supporting the N.F.L. players’ right to protest during the anthem.
“People don’t get mad when people are shot or killed, but they’re getting mad because a football player is kneeling or raising a fist,” said Melendez, who is African-American. “The double standard is crazy.”