American football player Colin Kaepernick was loudly booed on Thursday after once again refusing to stand during a rendition of the US national anthem.

The San Francisco 49ers quarterback has triggered debate in the United States over his decision to sit during the playing of The Star-Spangled Banner in a protest he says is aimed at drawing attention to the plight of blacks in the country.

The 28-year-old crouched on one knee during the anthem on Thursday as the 49ers played the San Diego Chargers at the Qualcomm Stadium in Southern California.

The latest protest came on a night when the Chargers honoured US military personnel in an annual "Salute to the Military".

San Diego has a large population of army personnel and is home to a large naval base.

READ MORE: Kaepernick anthem snub highlights 'structural racism'

After the rendition of the anthem - which was sung by a US soldier - Kaepernick was booed relentlessly by the crowd every time he took a snap in the opening plays of the game.

The jeers, though, did little to deter Kaepernick, with the quarterback leading the team on a smooth opening drive for an early touchdown to put the 49ers ahead.

The player's protest has divided opinion. Some have praised it as peaceful and powerful, while others say he is unpatriotic.

Over the past week, a photograph emerged of him wearing a pair of socks that bore images of cartoon pigs wearing police uniforms.

Kaepernick explained the photograph on his Instagram account: "I wore these socks, in the past, because the rogue cops that are allowed to hold positions in police departments, not only put the community in danger, but also put the cops that have the right intentions in danger by creating an environment of tension and mistrust."

He added that he has relatives in the police.

Focusing on his socks, "which were worn before I took my public stance ... distract from the real issues", he said.

Trump: Protest a 'terrible thing'

Republican presidential nominee Donald Trump described the protest as a "terrible thing" and suggested the player should leave the US.

"Maybe he should find a country that works better for him, let him try, it's not gonna happen," Trump said.

Kaepernick's stance has won applause from veteran civil rights campaigners and sporting icons including Tommie Smith, a sprinter who was ostracised for his clenched-fist salute alongside John Carlos at the 1968 Mexico City Olympic Games.

"He's just speaking out [but] he used a platform that many Americans don't agree with," Smith said on Tuesday.

"He's being vilified in how he brings the truth out. I support him because he's bringing the truth out - regardless of how done. If it's not done violently, at least he should be heard."

When he began his protest, Kaepernick said he refused to stand for the national anthem because the US "oppresses black people and people of colour".

Source: Agencies