By Amy Tennery

NEW YORK (Reuters) - NFL fans turned out in force on Twitter on Friday to heap scorn on Colin Kaepernick and two other players who refused to stand during the playing of the U.S. national anthem in a second week of pre-game protests, but some fellow players offered support.

Much of the criticism was directed at Kaepernick, the San Francisco 49er quarterback who made good on his vow to carry on with his protest against racial injustice and police brutality before his team's Thursday game against the San Diego Chargers.

A week earlier Kaepernick raised the ire of many fans when he remained seated through the traditional rendition of "The Star-Spangled Banner" before his team's game against the Green Bay Packers. Many Americans saw his gesture as a sign of disrespect to the flag.

Kaepernick is the latest professional athlete to use his celebrity to call attention to the issue of the mistreatment of minority groups by law enforcement. Over the past two years, the killings of a series of African-Americans in cities across the country has triggered protests and given rise to the Black Lives Matter movement.

"I am a veteran and asking all veterans & fellow Americans to boycott NFL until Kapernick & all who disrespect America are gone," tweeted Donna Hallabuk.

"You think Colin Kaepernick realizes there is an American Flag on the back of his helmet?," wrote Twitter user Josh Massey. "Shouldn't he play without one to take a stand?"

On Thursday, Seattle Seahawks cornerback Jeremy Lane and 49ers safety Eric Reid made an apparent show of solidarity with Kaepernick by declining to stand for the anthem ahead of their games, and some Twitter users took exception.

"Jeremy lane ... just like that dumbass 49er QB, you just disrespected every veteran and everybody that lives in the us," tweeted a Seahawks fan using the handle "slinny."

Meanwhile, an online petition demanding the National Football League punish Kaepernick for his protest attracted over 53,000 signatures by Friday.

"His actions continue to embarrass all professional sports and bring reproach on his team," the petition reads.

The league has issued a statement saying it encourages players to stand during the anthem but doesn't require them to do so, while the 49ers have framed the issue in terms of the right of free expression enjoyed by all Americans.

Some NFL players have rallied behind Kaepernick online.

Russell Okung, an offensive tackle on the Denver Broncos, said he appreciated Kaepernick's efforts to call attention to police violence against African-Americans.

"Colin Kaepernick, know that we see you, man," Okung wrote on The Player's Tribune, an online forum for professional athletes. "Thank you for reigniting the conversation and the movement for change."

Nate Boyer, a former U.S. Army Green Beret and a long-snapper currently listed as a free agent, posted a photo on Twitter of himself standing with Kaepernick with the caption, "Thanks for the invite brother... Good talk. Let's just keep moving forward. This is what America should be all about."

(Reporting by Amy Tennery in New York; Editing by Frank McGurty and James Dalgleish)