By Marty Graham

SAN DIEGO (Reuters) - All eyes will be on San Francisco 49ers quarterback Colin Kaepernick at a Thursday night football game to see if he will sit again during the U.S. national anthem to protest racial injustice in the United States.

Kaepernick, 28, has drawn both strong support and fierce criticism for refusing to stand during "The Star-Spangled Banner" before the 49ers' National Football League preseason games this year.

Kaepernick, who led San Francisco to the 2013 Super Bowl but has since been demoted to backup, has said he would not show pride in a flag for a country that oppresses people of color, citing police brutality. He is the latest black athlete to use the arena as a national platform for protest against racial injustice.

The use of force by police against African-Americans in cities such as Ferguson, Missouri, Baltimore and New York has sparked periodic and sometimes destructive protests in the past two years and prompted the rise of the Black Lives Matter movement.

Kaepernick had been sitting out the national anthem in San Francisco's first three preseason games but it was not widely noticed until last Friday's game against Green Bay.

He will be under great scrutiny at the game in San Diego, where the Chargers will be putting on their annual "Salute to the Military." Ceremonies will feature 240 military personnel presenting a giant American flag, a remembrance of the Vietnam War and a patriotic fireworks show, the Chargers said.

The 49ers, who unsuccessfully tried to trade Kaepernick since his demotion, have supported his protest, as have many others, including NFL Hall of Fame running back Jim Brown and National Basketball Association legend Kareem Abdul-Jabbar.

But the demonstration also has drawn fierce criticism from a wide variety of commentators, including the San Francisco Police Officer's Association, which in a statement called him "misinformed" and demanded an apology.

San Francisco's former police chief resigned in May under pressure from the city's mayor following the second of two recent high-profile police killings of black suspects and amid criticism over racist text messages sent by officers in the department.

Photos on sports websites and Twitter on Thursday showed Kaepernick wearing socks patterned with images of pigs with what appear to be police hats. News media reports indicated the photos were taken in mid-August, but the images sparked criticism from some social media users.

In a statement posted to his Instagram page, Kaepernick said he has relatives and friends who are police officers and that he had worn the socks in the past to call attention to "rogue cops."

Two seasons ago, players on the NFL's St. Louis Rams entered the stadium for a home game with their hands raised, a reference to the "Hands Up, Don't Shoot" slogan adopted by protesters in demonstrations against the shooting of an unarmed black teenager in nearby Ferguson.

More recently, players for the Women's National Basketball Association's Minnesota Lynx took the court last month with warmup shirts that said "Black Lives Matter."

The 49ers' Thursday away game against the San Diego Chargers is the team's last preseason contest before the regular season begins.

(Reporting by Marty Graham in San Diego and Rory Carroll in San Francisco, Amy Tennery in New York; Writing by Curtis Skinner; Editing by Jonathan Oatis and Bill Trott)