An American football player has refused to stand for the national anthem in protest of racial injustice against black people and ethnic minorities in the United States.

Colin Kaepernick, who plays with San Francisco 49ers, remained seated as the anthem was played ahead of a pre-season game against the Green Bay Packers.

The quarterback then faced boos from fans when he took to the field at Levi's Stadium.

Kaepernick's actions also prompted reaction on social media with some users supporting his stance while others criticised his "disrespect".

Sam White pointed out the irony that people that are angry at the player for sitting down during the anthem are not angry about "the injustice" he is protesting.

In an interview with NFL Media after the game, the 28-year-old, who is a supporter of the Black Lives Matter movement, defended his actions.

"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 said.

"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 in an apparent reference to police killings of black people.

The 49ers later issued a statement about Kaepernick's decision in which they said they "recognise the right of an individual to choose and participate, or not, in our celebration of the national anthem."

"Players are encouraged but not required to stand during the playing of the national anthem," the NFL added.

Kaepernick joins other high-profile athletes who have spoken out about police killings of unarmed black men and other issues affecting minorities, including basketball stars LeBron James and Carmelo Anthony and Michael Jordan