Kansas City Chiefs fans booed the reigning Super Bowl champions and the Houston Texans as the two teams lined up for a "moment of unity" ahead of Thursday's NFL season-opener at Arrowhead Stadium.
Scattered booing could be clearly heard coming from sections of the reduced-capacity crowd of 15,985 allowed into the game as players from both the Chiefs and Texans linked arms for the pre-game gesture near halfway.
Before the ritual, both teams had taken contrasting approaches to the playing of the US national anthem and a rendition of "Lift Every Voice and Sing" a song regarded as the black national anthem.
Texans players remained in their locker room for both anthems while the Chiefs players stood on the field.
One Chiefs player, defensive end Alex Okafor, dropped to one knee during the playing of the US national anthem.
The booing was greeted with dismay following the Chiefs 34-20 win.
"The moment of unity I personally thought was good," Texans star J.J. Watt told NFL Media. "I mean the booing during that moment was unfortunate. I don't fully understand that. There was no flag involved. There was nothing involved other than two teams coming together to show unity."
Kansas City mayor Quinton Lucas, who is black, said the catcalls did not reflect the Missouri city.
"We're a good city of good people," Lucas wrote on Twitter. "I heard boos too. But we also have hundreds of thousands more around here who respect the message the players are sharing."
Player protests have divided the NFL and its followers since former San Francisco 49ers quarterback Colin Kaepernick took up the cause against racial injustice and police brutality in 2016.
Kaepernick was widely vilified for his protests and has effectively been shut out of the NFL since being released by the 49ers in 2017.
The NFL initially opposed Kaepernick's protests, with league commissioner Roger Goodell saying players should stand for the US anthem.
However after months of protests across the United States following the death of George Floyd during his arrest by police in Minneapolis in May, the NFL relaxed its rules and said protests would be allowed.
Goodell has said he now regrets the league's failure to support Kaepernick and admits that the quarterback's protests had been mischaracterised as unpatriotic.
In June, Goodell issued a statement saying the league had been wrong and saying the NFL believed "black lives matter".
Goodell's statement followed calls for an apology led by Chiefs quarterback Patrick Mahomes and other players.
This season the NFL has launched a number of initiatives to demonstrate its support for activists campaigning against racial injustice.
End zones across the league will bear the words "End Racism" and "It Takes All of Us", while players will be allowed to wear helmet stickers featuring the names of victims of racism.