The NFL, on Sunday, is scrambling  to handle player demonstrations during the national anthem at opening games across the U.S.

The protests, initially sparked last month by San Francisco 49ers quarterback Colin Kaepernick, coincide with the 15th anniversary of the September 11 terror attacks.

While the NFL commemorates military personnel, first responders and two U.S. presidents at games, football teams including the Seattle Seahawks and Kansas City Chiefs sent symbols of protest.

Prior to their game against the Chargers on Sunday, the entire Chiefs team stood with their arms linked.

"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," the Chiefs said in a statement. Cornerback Marcus Peters also raised his right fist.

NFL Player's Association spokesman George Atallah said that players can’t violate the dress code—for example, by writing on their helmets—and they can’t interrupt the game. Apart from that, Mr. Atallah said, players can express themselves as they choose.

“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 of his decision to first sit, then subsequently kneel during the anthem at pre-season games.

Since then, several other players and teams across the nation on the high school, college, and professional level have joined Kaepernick in taking one knee for the anthem, including members of the Miami Dolphins, sparking outrage from fans.