The interim head of the NAACP Wednesday requested a meeting with NFL Commissioner Roger Goodell to discuss why free agent quarterback Colin Kaepernick has not been signed by any team.

Kaepernick attracted national attention last season as a member of the San Francisco 49ers when he refused to stand for "The Star-Spangled Banner" to protest police brutality against African-Americans. Kaepernick attracted further controversy when he praised Cuban dictator Fidel Castro, as well as the Black Panthers and the Black Lives Matter movement.

In the letter to Goodell, NAACP Interim President Derrick Johnson claimed Kaepernick was being blackballed for his advocacy.

"As outlined in your office's public statement, [Kaepernick's] act of dissent is well within the National Football League's stated bylaws," Johnson wrote. "Yet, as the NFL season quickly approaches, Mr. Kaepernick has spent an unprecedented amount of time as a free agent, and it is becoming increasingly apparent that this is no sheer coincidence."

"No player should be victimized and discriminated against because of his exercise of free speech — to do so is in violation of his rights under the Constitution and the NFL's own regulations," Johnson added.

In an email to Fox News, NFL spokesman Brian McCarthy said that the league had yet to receive the NAACP's letter and "would respond at the appropriate time."

Wednesday evening, more than 1,000 people demonstrated in support of the quarterback outside the NFL's headquarters in midtown Manhattan.

Protesters, many wearing jerseys bearing Kaepernick's name, chanted "Boycott! Boycott!" Women's March organizer Tamika Mallory, addressing football fans, said, "I don't care how long you've been watching football, if they don't stand up for your children, turn the damn TV off."

Political commentator Symone Sanders said, "We are all standing with Kaepernick. It is time for the NFL to put up or shut up."

Some other players followed Kaepernick's actions last season, and some are doing so in this year's pre-season. On Monday, a group of Cleveland Browns players prayed in silent protest during the national anthem. Among those kneeling was a white player, Seth DeValve. Another white player, Britton Colquitt, did not kneel but kept his hand on the shoulder of a teammate who did.

That protest earned the ire of an Ohio Supreme Court justice, the lone Democrat holding an Ohio statewide office. Justice Bill O'Neill wrote on Facebook that he wouldn't attend any games at which "draft dodging millionaire athletes disrespect the veterans who earned them the right to be on that field."

"Shame on you all," he said.

In his six seasons with the 49ers, Kaepernick started 58 games and compiled a career record of 28-30 while accounting for 85 all-purpose touchdowns and throwing just 30 interceptions. In 2012, Kaepernick started the final seven games of the regular season and led the 49ers to Super Bowl 46, where they lost 34-31 to the Baltimore Ravens.

The Associated Press contributed to this report.