NFL players began protesting in earnest on Sunday against President Trump’s insistence that team owners fire players who kneel during the national anthem and that fans boycott those players’ teams.

During the first game of the day, which took place in London between the Baltimore Ravens and the Jacksonville Jaguars, players on both sides knelt during the anthem while others locked arms. Jaguars owner Shahid Khan, who donated $1,000,000 to Trump’s inaugural committee, had his armed interlocked with his players on the field, as did Ravens coach John Harbaugh.

The controversy stems from the rally Trump held in Alabama on Friday night, when the president brought up the players’ protests seemingly at random.

“Wouldn’t you love to see one of these NFL owners, when somebody disrespects our flag, to say, ‘Get that son of a bitch off the field right now, out. He’s fired. He’s fired!’” Trump mused, referencing in part quarterback Colin Kaepernick’s long-running protest of treatment of minorities in America.

Trump kept the controversy alive the following day on Twitter, culminating in a plea that NFL fans collectively boycott games until the players who protest are fired.

Later Sunday, Pittsburgh Steelers players remained in the locker room while the national anthem was playing ahead of their game against the Chicago Bears. Steelers coach Mike Tomlin stood on the sideline by himself. Offensive tackle Alejandro Villanueva, who served three tours of duty in Afghanistan as an Army Ranger, stood outside the tunnel during the anthem.

The Seattle Seahawks also boycotted the national anthem. The team said in a statement that “we will not stand for the injustice that has plagued people of color in this country,” adding: “We unite to oppose those that would deny our most basic freedoms.”

Even some of Trump’s most prominent boosters in the professional sports industry criticized his remarks. Robert Kraft, who has been friends with Trump for years and has supported him politically, said on Sunday that he was “deeply disappointed” in the “tone” of Trump’s comments, and praised his players for attempting to “peacefully affect social change.”

Former Buffalo Bills coach Rex Ryan, who also supported Trump, said on Sunday he was “appalled” at what the president said.

“I’m pissed off, I’ll be honest with you,” Ryan said on ESPN. “Calling our players SOBs and all that kind of stuff? That’s not the men that I know. Men that I know in the locker room, I’m proud of. I’m proud to be associated with those people.”

Treasury Secretary Steven Mnuchin defended Trump’s comments on Sunday morning, casting the protests as disrespectful of the military and of the American flag.

“Players have the right for free speech off the field. On the field, this is about respect for lots of people,” Mnuchin said on CNN’s State of the Union. “For as long as I can remember, people have stood in honor of the country. This isn’t about politics. If people want to talk politics off the field when they’re not working for the NFL, they have the absolute right to do that.”

The controversy has spread beyond the football field. On Saturday night, Oakland Athletics catcher Bruce Maxwell became the first Major League Baseball player to kneel during the national anthem, saying Trump shouldn’t “ single out NFL players for doing this.”

Trump responded to the protests in a tweet on Sunday, applauding those who were locking arms but blasting the players who were kneeling. But the players who were joined at the arms were acting in solidarity with those who were kneeling.

“Great solidarity for our National Anthem and for our Country. Standing with locked arms is good, kneeling is not acceptable. Bad ratings!” Trump tweeted.