(CNN) President Donald Trump railed against NFL players who protested at Thursday night's preseason games, saying the players are outraged about "something that most of them are unable to define."

"The NFL players are at it again - taking a knee when they should be standing proudly for the National Anthem. Numerous players, from different teams, wanted to show their 'outrage' at something that most of them are unable to define. They make a fortune doing what they love," he tweeted from his Bedminster golf club on Friday morning, the ninth day of his vacation.

On Thursday, as the preseason games officially kicked off, several players took a knee, raised a fist, or did not take to the field while the National Anthem was played before a dozen preseason games across the country.

NFL players have said the protests, which began in 2016, are intended to draw attention to what they see as systematic bias against people of color.

"It's not necessarily about the anthem, that's where everybody's messing up," Tennessee Titans star defensive end Jurrell Casey told CNN last month. "The way that the justice system treats minorities is the issue that we have."

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