United States president Donald Trump condemned NFL players who protest the national anthem during a rally in Alabama on Friday.

While discussing "American values", Trump blasted the NFL's television ratings and pinned it on players protesting the anthem.

"Wouldn't you love to see one of these NFL owners, when somebody disrespects our flag, to say, 'Get that son of a b**** off the field right now, out, he's fired!' You know, some owner is going to do that," he said.

"He's going to say, 'That guy that disrespects our flag, he's fired.' And that owner, they don't know it. They don't know it. They're friends of mine, many of them. They don't know it. They'll be the most popular person, for a week. They'll be the most popular person in this country.

"The NFL ratings are down massively. Now the number one reason happens to be they like watching what's happening … with yours truly. They like what's happening.

"Because you know today if you hit too hard: 15 yards! Throw him out of the game! They had that last week. I watched for a couple minutes. Two guys, just really, beautiful tackle. Boom, 15 yards! The referee gets on television, his wife is sitting at home, she's so proud of him. They're ruining the game! They're ruining the game. That's what they want to do. They want to hit. They want to hit! It is hurting the game.

"But do you know what's hurting the game more than that? When people like yourselves turn on television and you see those people taking the knee when they're playing our great national anthem. The only thing you could do better is if you see it, even if it's one player, leave the stadium. I guarantee it will stop. Things will stop. Just pick up and leave. Pick up and leave. Not the same game anymore, anyway."