His backers include retired Seattle Seahawks star Marshawn Lynch who told TV show host Conan O'Brien: "With what's going on, I'd rather see him take a knee than stand up, put his hands up and get murdered.
"My take on it is ... fuck, they got to start somewhere. I just hope people open up their eyes and see that there's really a problem going on, and something needs to be done for it to stop."
In yet another incident of a black man being shot to death by police, the unarmed Terence Crutcher died in Tulsa Oklahoma last week. Police video showed him walking away from officers with his hands in the air. The officer who shot him has been put on paid leave.
Kaepernick said: "His car was broken down, he was looking for help and he got murdered.
"That's a perfect example of what this is about. I think it will be very telling what happens with the officers that killed him because everybody's eyes will be on this."
The Guardian reported that other NFL players have sat, knelt or raised fists during the Star-Spangled Banner. Football World Cup winner Megan Rapinoe has chosen to kneel before US international soccer games. Many high school and college players have also joined the protest.
"Do I kneel forever? I don't know, probably not," Rapinoe, 31, told ESPN. "But I think until I can feel like I'm being more effective in other ways, then this seems appropriate to me."
Among the detractors are Ted Cruz, the former presidential candidate, who said: "To all the athletes who have made millions in America's freedom: stop insulting the flag, our nation, our heroes."