By Amy Tennery

NEW YORK (Reuters) - San Francisco 49ers quarterback Colin Kaepernick's sideline protest during the playing of "The Star-Spangled Banner" at a National Football League pre-season game last week has generated criticism from current and former NFL stars.

Kaepernick, 28, sat during the national anthem on Friday in a protest against racial injustice in the United States, a move that prompted boos from some fans.

Former Pittsburgh Steelers wide receiver Hines Ward wrote in an Instagram post Tuesday that while he can "respect" Kaepernick's cause, he doesn't "agree with the method he chose."

"If you want to make a point or take a stand, go straight after the root of that cause," wrote Ward. "Don't disrespect the whole country or the organization that's paying you millions of dollars."

Jerry Rice, a member of the Pro Football Hall of Fame who won three Super Bowl championships while with the 49ers, wrote on Twitter Monday that Kaepernick should not "disrespect the flag."

"All lives matter. So much going on in this world today. Can we all just get along!" tweeted the former wide receiver and 20-year veteran of the league, adding that he respects Kaepernick's "stance."

New Orleans' Saints quarterback Drew Brees tweeted Monday, "I agree with his protest, I DON'T agree w his METHOD."

Rodney Harrison, a former safety for the San Diego Chargers and the New England Patriots, said on a Houston sports talk radio show Tuesday Kaepernick is "not black" and cannot understand the prejudice that black men face.

Harrison's remarks came under a barrage of criticism and he later apologized in a series of posts on Twitter, saying that he "should not have called Colin Kaepernick’s race into question."

Kaepernick has also attracted some unreserved support.

Hall of Famer Jim Brown said on the TV program "NFL Total Access" that he supports Kaepernick's stance.

"I think Pandora's box is open," Brown said. "I'm very happy that it is."

National Basketball Association legend Kareem Abdul-Jabbar offered a full-throated endorsement of Kaepernick's protest in a Washington Post editorial in which he called the failure to address deep-seated racism in the United States "what’s really un-American here."

When reached by e-mail for comment, NFL spokesman Brian McCarthy said that NFL players "are encouraged but not required to stand" for the national anthem.

The San Francisco 49ers, which previously released a statement in support of Kaepernick, declined to comment further.

The team is scheduled to play Thursday against the San Diego Chargers in its final pre-season matchup.

(Reporting By Amy Tennery; Editing by Cynthia Osterman, Andrew Hay and David Gregorio)