More than 1,000 people, many wearing jerseys bearing Kaepernick's name, crowded the steps outside the NFL's midtown Manhattan offices.
Supporters say he is being blackballed for his advocacy, but some critics say he should not have sat or kneeled during the anthem or contend his lack of a job is more about his on-field talent.
The civil rights organization's interim president, Derrick Johnson, said in a letter to the NFL's commissioner, Roger Goodell, that it's apparently "no sheer coincidence" that Kaepernick isn't on a roster.
"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 said.
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."
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