President Donald Trump reignited his feud with the National Football League on Friday, blasting football players who protested during preseason games the prior evening for "taking a knee when they should be standing proudly" and suggested that protesters should be "suspended without pay."

"The NFL players are at it again - taking a knee when they should be standing proudly for the National Anthem," Trump tweeted. "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...........Be happy, be cool!" Trump wrote. "A football game, that fans are paying soooo much money to watch and enjoy, is no place to protest. Most of that money goes to the players anyway."

"Find another way to protest. Stand proudly for your National Anthem or be Suspended Without Pay!" he added.

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...... — Donald J. Trump (@realDonaldTrump) August 10, 2018

.....Be happy, be cool! A football game, that fans are paying soooo much money to watch and enjoy, is no place to protest. Most of that money goes to the players anyway. Find another way to protest. Stand proudly for your National Anthem or be Suspended Without Pay! — Donald J. Trump (@realDonaldTrump) August 10, 2018

On Thursday night, over the course of 12 NFL preseason games, several NFL players took a knee, raised their fists or stayed off the field as the national anthem played before the start of the game. Trump has attacked those kinds of protests for more than a year, even saying at one point that players who don’t stand for the national anthem "maybe shouldn't be in the country."

In May, the NFL announced a policy that all players who are on the field when the national anthem is heard must stand, or could remain in the locker room without penalty, but the policy was later put on hold. In July, the NFL Players Association and the league said in a joint statement that they agreed to halt enforcement of rules regarding the new national anthem policy while the two sides work on a resolution.

The controversy over players who kneel during "The Star-Spangled Banner" has raged since 2016, when Colin Kaepernick, then a quarterback with the 49ers, first refused to stand as a lone protest against police brutality, particularly toward black Americans, and racial oppression. As the peaceful protests, conducted predominantly by black athletes, expanded, Trump began lashing out, criticizing the kneeling as “disgraceful" and calling those who kneel "sons of bitches."

Contrary to Trump's claim Friday that players have been "unable to define" what they're protesting, many athletes have repeatedly outlined their reasoning.

One of the players who protested by raising his fist Thursday night, Philadelphia Eagles safety Malcolm Jenkins, told NBC News' Lester Holt in June that he chose to participate in the protests last season because "I'm fighting for Americans and citizens that have been disenfranchised, that have been systemically oppressed for centuries."

And ahead of Thursday's games, Jenkins tweeted, "Before we enjoy this game lets take some time to ponder that more than 60 percent of the prison population are people of color," adding that, "The NFL is made up of 70 percent African-Americans. What you witness on the field does not represent the reality of everyday America. We are the anomalies..."

Former 49ers safety Eric Reid, who joined his former teammate Kaepernick by kneeling in 2016, wrote an op-ed for The New York Times in 2017 explaining that he was protesting police brutality and "systemic oppression against people of color."

Kaepernick and Reid remain unsigned free agents and have filed grievances against the NFL and its owners, accusing them of colluding.

Players who protested during Thursday night's games included Jenkins, who raised a fist during the anthem before the team’s game against the Pittsburgh Steelers; Miami Dolphins' wide receivers Kenny Stills and Albert Wilson, who kneeled during the anthem; and Dolphins’ defensive end Robert Quinn, who stood and raised his right fist during the team’s exhibition game against the Tampa Bay Buccaneers.

In addition, Seattle Seahawks' left tackle Duane Brown and defensive linemen Quinton Jefferson and Branden Jackson stayed in the locker room and were not on the field during the anthem before the Seahawks’ game against the Indianapolis Colts. Jacksonville Jaguars players Telvin Smith, Jalen Ramsey, Leonard Fournette and T.J. Yeldon remained in the tunnel when the national anthem played before the team's home game against the New Orleans Saints.