Kaepernick sparked a firestorm of criticism when he sat or kneeled during “The Star-Spangled Banner” at NFL pre-season games, but the latest protests came on the first big Sunday of the regular season.

He has said he is protesting US racial discrimination and police brutality in the wake of multiple high-profile fatal shootings of black men by law enforcement — most often by white police officers.

On Sunday, four members of the Miami Dolphins kneeled during the anthem while Kansas City Chiefs cornerback Marcus Peters raised his right black-gloved fist — a protest that evoked memories of American runners Tommie Smith and John Carlos on the medal podium at the 1968 Mexico City Olympics.

“What was going on in law enforcement, it does need to change,” Peters said Friday. 

“It does need to change for all, equal opportunities for everybody, not just us as black Americans. (…)  I don’t think nothing is being done about it.”

The Chiefs had linked arms in a sign of unity but Peters, who was on the right end of the line of players, raised his clenched fist.

In Seattle, where the Dolphins were taking on the Seahawks, Dolphins running back Arian Foster and teammates Kenny Stills, Michael Thomas and Jelani Jenkins kneeled on the sidelines during the US anthem.

Host Seahawks players linked arms during the anthem, including cornerback Jeremy Lane, who had taken a knee at a pre-season game.

NFL players, many of them African-Americans, also joined US first responders and military personnel across the league in pre-game tributes on the anniversary of the attacks on New York, Washington and Pennsylvania.

Video messages marking the 9/11 anniversary from US President Barack Obama and his predecessor George W. Bush — who was in office when the attacks took place — were played in NFL stadiums.

Kaepernick and the 49ers do not begin the season until Monday at home against the Los Angeles Rams.

– Players ‘have rights’ -US Vice President Joe Biden was among those holding a huge flag before the Philadelphia Eagles’ home victory over the Cleveland Browns, where a “U-S-A, U-S-A” chant broke out.

New York Jets players joined first responders in holding a huge American flag before their home victory over the Buffalo Bills.

Bush performed the pre-game coin toss as the Dallas Cowboys hosted the New York Giants, accompanied by his wife Laura and two New York police officers who were at Ground Zero in 2001.

“We hold the victims of 9/11 and their families in our hearts and on this 15th anniversary, we lift them up in our prayers,” Bush said in a statement.

The Giants called tails, the special coin side featuring the Twin Towers, but the coin flipped by Bush landed heads up, featuring the Lone Star logo of the Cowboys.

At some stadiums, US military airplanes flew a “Missing Man” formation over the venues, a nod to nearly 3,000 people who lost their lives 15 years ago.

NFL commissioner Roger Goodell told NBC that players should be allowed to protest — even on the anniversary of the deadly 9/11 attacks — but should also respect those who have sacrificed for America.

“They have rights and we have to respect that,” he said. 

“I support our players speaking out on issues that need to be changed in society. But that’s what the focus should be on, the changes he wants to see in the society.

“What I do believe, though, is the respect for our country, the people who fought for those freedoms and values, the people who protect us here and abroad — those are very important. We’re a patriotic league.”

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