Washington (CNN) President Donald Trump responded to protests across the National Football League on Sunday over his remarks about players taking a knee during the national anthem, expressing approval for those who locked arms, but not those who knelt.

"Great solidarity for our National Anthem and for our Country. Standing with locked arms is good, kneeling is not acceptable. Bad ratings!" the President tweeted Sunday afternoon.

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

Across the country, dozens of NFL players have been taking a knee during the playing of the national anthem before their Sunday games. Their protests come as Trump renewed his criticism of athletes taking a knee during the national anthem in protest over perceived social injustices.

On Sunday morning, Trump slammed the league's ratings in a post on Twitter and said again players should be fired or suspended for such protests -- reiterating remarks he made at a rally in Alabama Friday night.

If NFL fans refuse to go to games until players stop disrespecting our Flag & Country, you will see change take place fast. Fire or suspend!

...NFL attendance and ratings are WAY DOWN. Boring games yes, but many stay away because they love our country. League should back U.S.

The President inflamed tensions further Saturday by withdrawing a White House invitation to basketball star Stephen Curry of the Golden State Warriors. Curry had remarked at the Warriors' media day Friday that he doesn't want to attend the White House ceremony scheduled to commemorate the team's championship title.

"'By acting and not going, hopefully that will inspire some change when it comes to what we tolerate in this country and what is accepted and what we turn a blind eye to,'' Curry told reporters. ''It's not just the act of not going. There are things you have to do on the back end to actually push that message into motion."

Going to the White House is considered a great honor for a championship team.Stephen Curry is hesitating,therefore invitation is withdrawn! — Donald J. Trump (@realDonaldTrump) September 23, 2017

If a player wants the privilege of making millions of dollars in the NFL,or other leagues, he or she should not be allowed to disrespect.... — Donald J. Trump (@realDonaldTrump) September 23, 2017

...our Great American Flag (or Country) and should stand for the National Anthem. If not, YOU'RE FIRED. Find something else to do! — Donald J. Trump (@realDonaldTrump) September 23, 2017

At the first NFL game Sunday, which took place at London's Wembley Stadium, members of both the Jacksonville Jaguars and Baltimore Ravens took a knee while many other players and coaching staff stood and linked arms. Dozens more players on other teams knelt at their games Sunday afternoon.

Notably, almost the entire Pittsburgh Steelers roster stayed in their locker room during the national anthem.

The NFL announced Sunday morning that it will be dusting off a one-minute ad it produced for the Super Bowl earlier this year that originally was produced to "demonstrate the power of football to bring people together," the league said.

In response to Trump's remarks in Alabama, Robert Kraft, of close friend of Trump's and owner of the New England Patriots, issued a statement Sunday morning saying: "I am deeply disappointed by the tone of the comments made by the President on Friday. I am proud to be associated with so many players who make such tremendous contributions in positively impacting our communities."

NFL players began to kneel during the national anthem last year, when then-San Francisco 49ers quarterback Colin Kaepernick refused to stand during "The Star-Spangled Banner" before kickoff, sparking both support and backlash.

Kaepernick, who is currently without a team, told NFL Media last August : "I am not going to stand up to show pride in a flag for a country that oppresses black people and people of color."

On Sunday morning, Treasury Secretary Steve Mnuchin appeared on CNN's State of the Union with Jake Tapper and said that NFL owners should decide on a rule on how to handle players who kneel in protest during the national anthem.

"The owners should meet and decide on this rule the way they decide on any other rule," Mnuchin said.