Several NFL players who protested during the US national anthem before pre-season games have been criticised by President Donald Trump.
Ex-49ers quarterback Colin Kaepernick was the first to protest in 2016 and some players raised a fist and others kneeled at three games on Thursday.
The NFL says plans to fine players for anthem protests have been put on hold.
"Find another way to protest, stand proudly for your national anthem or be suspended without pay," tweeted Trump.
"Numerous players, from different teams, wanted to show their 'outrage' at something that most of them are unable to define.
"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."
Players who refused to stand during The Star-Spangled Banner have said the protests were against police brutality of African Americans and racial inequality, and the demonstrations have previously been criticised by Trump.
On Thursday, Philadelphia Eagles' Malcolm Jenkins and De'Vante Bausby, San Francisco 49ers' Marquise Goodwin and Miami Dolphins' Robert Quinn raised a fist, while his team-mates Kenny Stills and Albert Wilson kneeled.
"I just think it's important to keep this conversation going, that we don't let it get stagnant," said Jenkins after the Eagles' 31-14 loss to Pittsburgh Steelers.
"Everybody's kind of waiting to see what the league is going to do. It's just my personal decision to keep these things at the forefront."
The NFL says its anthem policy is on hold while it holds discussions with the NFL Players Association.
It reiterated that players who do not wish to stand during the anthem should stay in the changing room.
The governing body said: "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.
"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."