United States (US) President Donald Trump says National Football League (NFL) owners should fire players who kneel during the national anthem. He's also encouraging spectators to walk out in protest.

In an extended riff during a freewheeling rally speech in Alabama Friday night, Trump also bemoaned that football games have become less violent.

“They're ruining the game,” he complained.

Several athletes, including a handful of NFL players, have refused to stand during “The Star-Spangled Banner” to protest of the treatment of blacks by police. Quarterback Colin Kaepernick, who started the trend last year when he played for the San Francisco 49ers, hasn't been signed by an NFL team for this season.

Trump says those players are disrespecting the flag and deserve to lose their jobs.

“That's a total disrespect of our heritage. That's a total disrespect of everything that we stand for,” he said, encouraging owners to act.

“Wouldn't you love to see one of these NFL owners, when somebody disrespects our flag, you'd say, 'Get that son of a bitch off the field right now. Out! He's fired,” Trump said to loud applause.

Trump also predicted that any owner who went through with his encouragement would become “the most popular person in this country” at least for a week.

Trump, who was in Alabama campaigning for Senator Luther Strange, also blamed a decline in NFL ratings on the nation's interest in “yours truly” as well as what he described as a decline in violence in the game.

He said players are being thrown out for aggressive tackles, and it's “not the same game.”

Over the past several seasons, the NFL and college football have increased penalties and enforcement for illegal hits to the head and for hitting defenceless players.

A July report on 202 former football players found evidence of a debilitating brain disease linked to repeated head blows in nearly all of them. The league has agreed to pay $1 billion to retired players who claimed it misled them about the concussion dangers of playing football.

During his campaign, Trump often expressed nostalgia for the “old days” claiming, for example, that protesters at his rallies would have been carried out on stretchers back then. He recently suggested police officers should be rougher with criminals and shouldn't protect their heads when pushing them into squad cars.

It's also not the first time he's raised the kneeling issue. Earlier this year he took credit for the fact that Kaepernick hadn't been signed.

Television ratings for the NFL have been slipping since the beginning of the 2016 season. The league and observers have blamed a combination of factors, including competing coverage of last year's presidential election, more viewers dropping cable television, fans' discomfort with the reports of head trauma and the anthem protests.

Ratings have been down even more in the early 2017 season, though broadcasters and the league have blamed the hurricanes that hit Florida and Texas. Still, the NFL remains by far the most popular televised sport in the United States.

Trump said the anthem protest was the top reason NFL viewership had waned.

“You know what's hurting the game?” he asked. “When people like yourselves turn on television and you see those people taking the knee when they're playing our great national anthem,” he said.

Trump encouraged his supporters to pick up and leave the stadium next time they spot a player failing to stand.

“I guarantee things will stop,” he said.