Players from the Portland Thorns and North Carolina Courage knelt during the playing of the US national anthem on Saturday in solidarity with the Black Lives Matter movement.

The protest has been repeated across the world of football in the weeks since Minneapolis resident George Floyd was killed through alleged excessive force on the part of officers during a police operation.

Bundesliga stars Jadon Sancho and Marcos Thuram were among the first to show their support of Black Lives Matter, while the Premier League also saw players wear the movement's name on their shirts in place of their own surnames.

Saturday's NWSL Challenge Cup match marked the return of professional sport in the US following lockdown due to the coronavirus pandemic and members of both teams 'took a knee', echoing the gesture first brought into the public eye by NFL quarterback Colin Kaepernick.

Players also wore Black Lives Matter shirts during the playing of the anthem, which was performed on a saxophone.

The issue of kneeling prior to sports matches has long been a focus of fevered discussion inside US sport, and was in fact banned by US Soccer in 2016 when national team star Megan Rapinoe performed the act in sympathy with Kaepernick.

Earlier in June, however, the governing body repealed its policy forcing players to stand, and the NWSL saluted the act in a message shared on its official Twitter account.

Among the most vehement critics of 'taking a knee' is President of the United States, Donald Trump, who has suggested in the past that doing so during the anthem should be a "deportable act".

Trump vowed to stop watching football due to the U-turn on kneeling, while Republican congressman Matt Gaetz went further, calling for the US national teams to be disbanded if they refuse to stand.

"I’d rather the US not have a soccer team than have a soccer team that won’t stand for the National Anthem," the Florida politician stated in a message that was shared by Trump.

"You shouldn’t get to play under our flag as our national team if you won’t stand when it is raised."