SOMERSET, N.J. -- President Donald Trump continued to criticize the NFL in tweets Sunday, after taking aim at top professional athletes in comments that brought swift condemnation Saturday from league executives and star players.

"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!" Mr. Trump tweeted Sunday.

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

Later on Sunday, Mr. Trump again took to Twitter -- after teams were seen locking arms and kneeling at their respective games.

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

He later added, after retweeting a post calling for a boycott of the NFL, "Courageous Patriots have fought and died for our great American Flag --- we MUST honor and respect it! MAKE AMERICA GREAT AGAIN!"

The tweets came after Mr. Trump denounced protests by NFL players and rescinded a White House invitation for NBA champion Stephen Curry.

Wading into thorny issues of race and politics, Mr. Trump's comments in a Friday night speech and a series of Saturday tweets drew sharp responses from some of the nation's top athletes, with LeBron James calling the president a "bum." Hours later, Major League Baseball saw its first player take a knee during the national anthem.

Mr. Trump started by announcing that Curry, the popular two-time MVP for the Golden State Warriors, would not be welcome at the White House for the commemorative visit traditionally made by championship teams.

"Going to the White House is considered a great honor for a championship team. Stephen Curry is hesitating, therefore invitation is withdrawn!" he tweeted early Saturday.

Later, Mr. Trump reiterated what he said at a rally in Alabama the previous night -- that NFL players who kneel for the national anthem should be fired, and called on NFL Commissioner Roger Goodell to tell them to stand. Goodell and several team owners criticized the comments.