Their actions, on the 15th anniversary of the 9/11 attacks, raised the ire of some celebrities.
On the first Sunday of the NFL season additional players showed signs of support for San Francisco 49ers quarterback Colin Kaepernick's call to end racial inequality.
Read: As Kaepernick Takes a Knee, More Athletes Refuse To Stand During National Anthem
Several Miami Dolphins players — Arian Foster, Jelani Jenkins, Michael Thomas and Kenny Stills — kneeled on the sidelines during the national anthem prior to their game against the Seattle Seahawks.
The Dolphins released a statement on Twitter following the controversial protest, saying: "We hope today’s events will continue a respectful and thoughtful dialogue in our community on unity, inclusiveness and togetherness."
— Miami Dolphins (@MiamiDolphins) September 11, 2016
In contrast, the Seahawks linked arms in unity.
Marcus Peters, a player with the Kansas City Chiefs and two New England Patriots players, Devin McCourty and Martellus Bennett, raised their fists in the air as the anthem played.
On Friday, Peters said during a press conference that what Kaepernick started is for "a great cause."
He added: "[Kaepernick] spoke up about something he felt he needed to speak up about. I salute him for that. I'm going to back him up."
But because Sunday's games coincided with the 15th anniversary of the September 11 attacks, the Twitterverse exploded with rage, viewing the players' actions as disrespectful.
One disgusted celebrity was supermodel Kate Upton, who tweeted: "Sitting or kneeling down during the national anthem is a disgrace to those people who have served and currently serve our country."
And she didn't stop there, posting additional rants about the protests.
This is unacceptable. You should be proud to be an American. Especially on 9/11 when we should support each other. https://t.co/qFozwaH9Bu
— Kate Upton (@KateUpton) September 11, 2016
Have every right to protest&should stand up for beliefs.Justice system IS corrupt.Not the time during anthem on 9/11 https://t.co/JH1J9gTusV
— Kate Upton (@KateUpton) September 12, 2016
She was joined by Parks and Recreation star Rob Lowe, who also took to Twitter, saying: “Any player who wants to boycott the anthem on 9/11 should be asked to remain in the locker room until kick off. It's not their moment.”
Prior to the games, a video message from former President George H. W. Bush and President Obama was played in stadiums around the country to commemorate the somber anniversary.
A portion of Obama's speech about remembering those who sacrificed all on 9/11 was booed by some in attendance.
More boos than cheers for Obama pic.twitter.com/hdTThQh8Ak
— Rob Crilly (@robcrilly) September 11, 2016
The crowed BOOED when Obama's recorded speech was introduced to the RAVENS crowd. #NflGirlfriends pic.twitter.com/mOyEiNKL4U
— Dabney Porte (@DabneyPorte) September 11, 2016
Earlier in the month, Obama defended Kaepernick's actions, saying it was his Constitutional right to protest the anthem.
The protesting of the national anthem, started by Kaepernick during preseason in August, has even spread to other sports.
Women’s soccer star Megan Rapinoe took a knee before a game with her professional team, the Washington Spirit, on September 8.
Read: Colin Kaepernick Dividing America as He Vows to Sit for National Anthem Again
Even a Navy sailor sat in solidarity with Kaepernick. As the anthem played at a Florida military base earlier in September, an unidentified sailor recorded herself sitting during the song and posted it online.
U.S. Navy spokeswoman Lt. Cdr. Katherine Meadows said in a statement that the sailor could face punishment.
"The Navy has identified the Sailor and her chain of command has been made aware of the video. Appropriate administrative actions are pending. Additionally, we routinely give training to Sailors on appropriate usage of social media and that they must abide by the Uniform Code of Justice at all times."
Watch: Kaepernick Jerseys Burned in Effigy After NFL Star Refuses to Stand for National
