Social justice took center stage on Sunday during the NFL's opening weekend. 

AP Photo/Stephen Brashear - apimages.com

Players across the country staged demonstrations to show solidarity with San Francisco 49ers quarterback Colin Kaepernick, whose silent protest during the preseason sparked a national conversation about the First Amendment and patriotism.

Kaepernick sat during the national anthem in an August preseason game to protest racial inequality in the United States.

“I am not going to stand up to show pride in a flag for a country that oppresses black people and people of color,” Kaepernick said in an August 27 interview with NFL.com. “To me, this is bigger than football, and it would be selfish on my part to look the other way. There are bodies in the street and people getting paid leave and getting away with murder.”

Kenny Stills, Michael Thomas, Arian Foster and Jelani Jenkins — players from the Miami Dolphins — each took a knee in protest Sunday.

The Kansas City Chiefs' Marcus Peters and three of Tennessee Titans raised their fists — a symbol of Black Power — during the anthem. All of the Chiefs teammates linked arms.

In a statement reported by the Huffington post, The Chiefs explained:

"After having a number of thoughtful discussions as a group regarding our representation during the national anthem, we decided collectively to lock arms as a sign of solidarity. It was our goal to be unified as a team and to be respectful of everyone’s opinions, and the remembrance of 9/11. It’s our job as professional athletes to make a positive impact on our communities and to be proactive when change is needed. Together we are going to continue to have conversations, educate ourselves and others on social issues and work with local law enforcement officials and leaders to make an impact on the Kansas City community."

Seattle Seahawks players and coaches stood, arms linked, on Sunday.

Even high school players have joined in.

These demonstrations follow Thursday's NFL season opener, when Denver Broncos linebacker Brandon Marshall knelt during the national anthem.

“The message is I’m against social injustice,” Marshall told NFL.com. “I’m not against the military or police or America at all. I’m against social injustice and I feel like this was the right thing to do.”