In a sign of solidarity with a fellow player, athletes in stadiums across the country joined a controversial protest against the US anthem as the National Football League season got underway.

Several football players refused to sing the Star Spangled Banner or stand in its honour as the song was sung as it traditionally is at the start of games.

The protest began in August at a pre-season game when mixed race San Francisco 49ers player Colin Kaeperneck refused to stand during the singing of the national anthem to protest against the oppression of blacks and the killing of unarmed African-Americans by white police officers in several US cities.

On the opening day of the official season Sunday (11 September) Kansas City Chiefs' cornerback Marcus Peters raised his fist in a sign of black power at the start of a home game against the San Diego Chargers. The rest of his teammates locked arms in a sign of unity, ABC News reported.

Four Miami Dolphins players in Washington knelt during the national anthem, while their rival Seattle Seahawks remained silent and locked arms during the hymn. Members of the Tennessee Titans also raised their fists before their game against the Minnesota Vikings, as did two New England Patriots before a game against the Arizona Cardinals.

Kaepernick was widely criticised after his initial protest by many who saw it for some reason as a criticism of the US armed forces.

"I am not going to stand up to show pride in a flag for a country that oppresses black people and people of colour," Kaepernick explained after the game. "To me, this is bigger than football and it would be selfish on my part to look the other way.

"There are bodies in the street and people getting paid leave and getting away with murder," he added referring to the police shootings of black men.

Republican presidential candidate Donald Trump suggested that Kaepernick should "find another country" to live in, and police threatened to boycott 49ers games. But President Obama said Kaepernick was exercising his "constitutional right" to protest.

As players were joining the protest, ESPN analyst Trent Dilfer lashed Kaepernick on TV.

"The big thing that hit me through all this was this is a backup quarterback whose job is to be quiet, and sit in the shadows and get the starter ready to play Week 1," Dilfer said on Sunday NFL Countdown. San Francisco 49ers outside linebacker Eli Harold called Dilfer an "idiot."

Kaepernick appears to no longer be weathering the storm alone.

"He spoke up about something he felt he needed to speak up about," said Peters of the Chiefs. "I salute him for that. I'm going to back him up."

Peters' raised fist was reminiscent of a similar protest by Tommie Smith and John Carlos, gold and bronze medalists after the 200-meter run at the 1968 Olympic Games on the winners' platform.

Even high school football players are backing Kaepernick, in some cases following the example of their coaches, notes Forbes magazine.

Kaepernick's 49ers are scheduled to host the Los Angeles Rams in a game Monday 12 September when he is expected to continue his protest.