By Frank Pingue

(Reuters) - The NFL said on Friday it will commemorate the 15th anniversary of the Sept. 11 attacks on the United States during games on Sunday, the same day when some players plan to protest during the U.S. national anthem.

Four players have so far opted to kneel during the anthem in a protest against social injustice, a controversial gesture that started during the preseason and one that many consider to be a sign of disrespect to the American flag.

San Francisco 49ers backup quarterback Colin Kaepernick began protests when he refused to stand for "The Star-Spangled Banner" and others have followed suit, most recently Denver linebacker Brandon Marshall ahead of Thursday's season opener.

The protests look set to continue, even on a day when the NFL recognizes the anniversary of the worst attack on American soil since Japanese forces bombed Pearl Harbor in 1941.

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

Seattle's receiver Doug Baldwin said this week he is considering sitting during the anthem ahead of Sunday's home game while team mate Bobby Wagner said the Seahawks have been talking about a group action.

According to the National Football League, fans across the country will see video messages from President Barack Obama and President George W. Bush prior to each of Sunday's 13 games on the first full day of action in the 2016 season.

First responders, community volunteers and members of the military will also be on the field for pregame activities and the playing of the national anthem, the league said in a statement.

A 9/11 decal will be placed on players' helmets while all team coaching staffs will be supplied with 9/11 lapel pins.

One of Kaepernick's preseason anthem protests coincided with "Salute the Military Night," which saw 240 sailors, Marines and soldiers present a U.S. flag and a pre-game parachute jump by retired Navy SEALS ahead of a game in San Diego.

Kaepernick, who has said his protest is not aimed at the military, applauded from the sidelines during a second-quarter salute to military members.

(Reporting by Frank Pingue in Toronto; Editing by Mark Lamport-Stokes)