Fellow players joined San Francisco 49er Colin Kaepernick in protesting about racial inequality on the NFL’s opening weekend, which also marked the anniversary of the 9/11 attacks.
Kaepernick’s refusal to stand for the national anthem at the start of each game in pre-season has divided opinion.
While many have backed him, others say he is disrespecting the American flag.
But several players raised fists or knelt to support Kaepernick’s aim to highlight oppression of black people.
Four Miami Dolphins players – Arian Foster, Jelani Jenkins, Michael Thomas, and Kenny Stills – knelt as the Star Spangled Banner played.
Their opponents, the Seattle Seahawks, locked arms as they stood.
Kansas City Chiefs players also locked arms to show unity, while Marcus Peters raised a black-gloved fist – a gesture reminiscent of US sprinters Tommie Smith and John Carlos as they stood on the podium following their 200m final at the 1968 Olympic Games in Mexico City.
In a statement, the Chiefs players said it was their goal to be “unified as a team and to be respectful of everyone’s opinions” as well as remember 9/11.
It continued: “It’s our job as professional athletes to make a positive impact on our communities and to be proactive when change is needed.”
Dolphins player Jelani Jenkins said: “I chose to get involved to see if I could create change, raise awareness.
“I want to make it clear that there is no disrespect to the military or to police officers. I’m not about that.
“I would like to keep moving forward in the right direction with everybody: equal rights, equal opportunity.
“It doesn’t seem that it’s happening. That’s why I took a stand.”
Seahawks players and staff had been in talks before their game to discuss how best to show support.
Wide receiver Doug Baldwin said they wanted to meet with the mayor of Seattle and local law enforcement.
“We know that there has to be change and progress, but you got to apply direction to change,” he said.
New England Patriots duo Martellus Bennett and Devin McCourty also held up their right arms, while team-mate Danny Amendola was seen to grab the unfurled American flag on the field during the anthem.
In Thursday’s season-opener between the Denver Broncos and the Carolina Panthers – a repeat of last season’s Super Bowl – Denver linebacker Brandon Marshall knelt during the national anthem.
Kaepernick’s San Francisco play the Los Angeles Rams on Monday.