President Trump on Friday mocked NFL players who were “at it again,” kneeling during the national anthem, and encouraged them to “Be happy, be cool!” since they make a “fortune doing what they love.”

Trump once again warned players to “Stand proudly” for the national anthem, or “be suspended Without Pay!”

“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,” Trump tweeted.

“They make a fortune doing what they love…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 the money goes to the players anyway. Find another way to protest. Stand proudly for your National Anthem or be Suspended Without Pay!”

The president’s tweet comes hours after NFL players protested during the anthem as the league began its first full week of the 2018 preseason.

Some players, like Robert Quinn of the Miami Dolphins, and the Philadelphia Eagles’ Malcolm Jenkins and De’Vante Bausby, raised their fists in the air during the anthem.

Wide receivers Kenny Stills and Albert Wilson of the Miami Dolphins knelt, while defensive end Michael Bennet spent the entirety of the anthem walking toward the bench.

The NFL, in May, modified its national anthem policy, prohibiting any sort of demonstrations for the 2018 season and making individual teams responsible for disciplining players who protest on the field.

The players' union filed a grievance about the policy change, and late last month, the new policy was put on hold while the NFL and NFL Players Association work on a resolution.

In statement released Thursday, the NFL said all players and team staff on the field were expected to stand for the flag or the anthem, reminding those who choose not to that they must remain in the locker rooms.

“The NFL has been engaged in constructive discussions with the NFL Players Association regarding the anthem and issues of equality and social justice that are of concern to many Americans,” the NFL statement said. “While those discussions continue, the NFL has agreed to delay implementing or enforcing any club work rules that could result in players being disciplined for their conduct during the performance of the anthem.”

The NFL added that they “remain committed to working with the players to identify solutions and to continue making progress on important social issues affecting our communities.”

Last season, dozens of NFL players participated in protests—sparked by former San Francisco’s 49ers quarterback Colin Kaepernick’s move to kneel during the anthem in protest of social injustice and racial inequality.

Trump, throughout the 2017 NFL season, blasted NFL protesters, condemning their actions as disrespectful to America and the flag.

Fox News’ Nicole Darrah, Elizabeth Zwirz, Stephen Sorace and Amy Lieu contributed to this report.