President Trump said Monday that football fans are demanding respect for the U.S. flag, and said the NFL must take steps to ensure that players stand, instead of taking a knee, when the national anthem is played.

"The issue of kneeling has nothing to do with race. It is about respect for our Country, Flag and National Anthem. NFL must respect this!" Trump tweeted Monday morning.

The issue of kneeling has nothing to do with race. It is about respect for our Country, Flag and National Anthem. NFL must respect this! — Donald J. Trump (@realDonaldTrump) September 25, 2017

More than 100 NFL players in various Sunday games chose to kneel during the anthem.

"Many people booed the players who kneeled yesterday (which was a small percentage of total). These are fans who demand respect for our Flag!" Trump tweeted Monday.

Many people booed the players who kneeled yesterday (which was a small percentage of total). These are fans who demand respect for our Flag! — Donald J. Trump (@realDonaldTrump) September 25, 2017

Trump praised NASCAR drivers and attendees at Sunday's race for not taking a knee during the national anthem.

"So proud of NASCAR and its supporters and fans. They won't put up with disrespecting our Country or our Flag — they said it loud and clear!" Trump added.

So proud of NASCAR and its supporters and fans. They won't put up with disrespecting our Country or our Flag - they said it loud and clear! — Donald J. Trump (@realDonaldTrump) September 25, 2017

At a rally Friday evening, Trump called for the firing of any professional athlete who kneels during the song to protest racism.

NFL Commissioner Roger Goodell criticized Trump for his comments, and Trump responded by saying the league was trying to "justify" players' poor behavior.

Trump also said the "White House never looked more beautiful" than last night, following his return from a week spent in New York City and Bedminster, N.J.