ATLANTA — President Trump, who has criticized professional football players for kneeling during the national anthem, stood on the field here on Monday as it played before the College Football Playoff championship game between the University of Alabama and the University of Georgia.

Flanked by members of the Reserve Officers’ Training Corps, Mr. Trump placed his right hand over his heart, just below an American flag pin on his lapel, and mouthed the words while the Zac Brown Band sang the anthem. Afterward, Mr. Trump quickly retreated to the security of a private box.

The president’s appearance at the game seemed intended to emphasize his furious critique of athletes for using football games as a place for protests. Neither team’s players went onto the field on Monday until after the anthem was played, as is standard at college football games.

Earlier in the day, during a speech to farmers in Nashville, Mr. Trump repeated his frustration with the players who knelt during N.F.L. games.